PROVISION OF DIFFERENT NETWORK USAGE ADVANCE SERVICES TO DIFFERENT CATEGORIES OF SUBSCRIBERS CROSS-REFERENCE(S) TO RELATED APPLICATIONS This application claims priority from South African provisional patent application number 2020/04133 filed on 7 July 2020 which is incorporated by reference herein. FIELD OF THE INVENTION This invention relates to a system and method for provision of different network usage advance services to different categories of subscribers. BACKGROUND TO THE INVENTION A prepaid mobile subscriber is typically required to pre-pay for any telephonic transaction conducted on a mobile telephone network via their mobile handset. Pre-payment is typically by purchasing airtime, which can then be used to transact on (or use) the mobile telephone network until depletion. There are however facilities available to prepaid mobile subscribers by way of which airtime and/or mobile bundles can be provisioned to the prepaid subscriber in advance of received payment (e.g. on credit). These advances (termed “network usage advances” herein) may be beneficial to prepaid subscribers in that such subscribers may continue to conduct telephonic transactions on the mobile telephone network even after their airtime and/or mobile bundles have been depleted. Typically, eligibility to receive such network usage advances is determined by credit scoring subscribers by evaluating various criteria associated with the relevant subscriber. However, traditional platforms for provisioning network usage advances to subscribers may lack the flexibility to enable provisioning of different network usage advance services to different categories of subscribers. There is accordingly scope for improvement. The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application. SUMMARY OF THE INVENTION In accordance with an aspect of the invention there is provided a computer-implemented method comprising: receiving a request to provision a network usage advance, the request being associated with a subscriber identifier and a service class, wherein a network usage advance enables usage of a mobile telephone network by a prepaid mobile subscriber in advance of received payment, and wherein different categories of subscribers are associated with different service classes which correspond to different network usage advance services; obtaining service class data associated with the service class; and, transmitting an instruction to cause provision of the network usage advance to an account associated with the subscriber identifier in accordance with the service class data, wherein the service class data is usable by an intelligent network of the mobile telephone network to provision different network usage advance services based on the service class data. The service class data may include one or more account identifiers associated with the service class and/or tariff data associated with the service class. The instruction may include the one or more account identifiers, wherein the instruction causes provision of the network usage advance to an account associated with one of the one or more account identifiers and subscriber identifier. The intelligent network may maintain different accounts for the same subscriber which are configured for provision of different network usage advance services. The instruction may include the tariff data, wherein instruction causes provision of the network usage advance to an account associated with the subscriber identifier and configuration of the intelligent network to apply a tariff in accordance with the tariff data. A first service class may be associated with a zero-fee credit service and a second service class may be associated with a fee-based credit service. The intelligent network may maintain, for each prepaid mobile subscriber, one or both of: one or more accounts associated with the first service class; and, one or more accounts associated with the second service class. At least one of the one or more accounts associated with the first service class may be configured for depletion in accordance with first tariff data. At least one of the one or more accounts associated with the second service class may be configured for depletion in accordance with second tariff data. The zero-fee credit service may be a network usage advance service by way of which the fee component is not charged upfront but is charged by way of the inflated tariff of consuming the given network usage advance. The method may include associating the prepaid mobile subscriber with the first service class or second service class based on credit data associated with the subscriber. The method may include dynamically updating the association of the prepaid mobile subscriber and service class based on changes in the credit data associated with the subscriber. The method may include transmitting a network usage advance offer to a mobile communication device associated with the subscriber identifier, wherein the offer includes information associated with the service class with which the prepaid mobile subscriber is associated. The offer may include information associated with the service class with which the subscriber is associated. The method may include determining particulars of the offer based on one or more of: a credit score, an outstanding network usage advance status, a network usage advance eligibility, a total credit limit and a service class assignment. Receiving the request may include receiving a subscriber originating request or an auto- generated request, wherein the auto-generated request is generated automatically in response to detecting a low balance condition in an account associated with the subscriber. The method may be conducted at a server computer and wherein the instruction is transmitted to the intelligent network of the mobile telephone network. A subscriber having a credit score being equal to or less than a predetermined low value or equal to or greater than a predetermined high value may be assigned to a first service class. A subscriber having a credit score being greater than the predetermined low value and less than the predetermined high value may be assigned to a second service class. A subscriber having a credit score being equal to or greater than the predetermined high value may be assigned to: the first service class if the subscriber is associated with an outstanding network usage advance status that indicates an outstanding network usage advance and if a total credit limit value with which the subscriber is associated is greater than zero; or, the second service class if the subscriber is associated with an outstanding network usage advance status that does not indicate an outstanding network usage advance or if a total credit limit value with which the subscriber is associated is equal to zero. In accordance with a further aspect of the invention there is provided a system comprising: a processor and a memory configured to provide computer program instructions to the processor to execute functions of components; a request receiving component for receiving a request to provision a network usage advance, the request being associated with a subscriber identifier and a service class, wherein a network usage advance enables usage of a mobile telephone network by a prepaid mobile subscriber in advance of received payment, and wherein different categories of subscribers are associated with different service classes which correspond to different network usage advance services; a service class data obtaining component for obtaining service class data associated with the service class; and, an instruction transmitting component for transmitting an instruction to cause provision of the network usage advance to an account associated with the subscriber identifier in accordance with the service class data, wherein the service class data is usable by an intelligent network of the mobile telephone network to provision different network usage advance services based on the service class data. A first service class may be associated with a zero-fee credit service and a second service class may be associated with a fee-based credit service. The system may include: a subscriber database in which a record associated with the prepaid mobile subscriber is stored, wherein the record includes the subscriber identifier and a service class assignment; and, a service class assignment data structure in which each service class is linked to corresponding service class data. The service class data obtaining component for obtaining service class data associated with the service class may be configured to: determine the service class with which the request is associated by querying the subscriber database for the subscriber record using the subscriber identifier; and, query the service class assignment data structure to obtain service class data associated with the service class. The system may include the intelligent network. The intelligent network may maintain, for each prepaid mobile subscriber, one or both of: one or more accounts associated with the first service class; and, one or more accounts associated with the second service class. At least one of the one or more accounts associated with the first service class may be configured for depletion in accordance with first tariff data. At least one of the one or more accounts associated with the second service class may be configured for depletion in accordance with second tariff data. The system may include a rating tariff table in which: at least one of the one or more accounts associated with the first service class is associated with first tariff data; and, at least one of the one or more accounts associated with the second service class is associated with second tariff data. The service class data may include one or more account identifiers and associated rules for provisioning in accordance with the service class such that provision of the network usage advance to one of the one or more accounts associated with the first service class causes application of the first tariff data and provision of the network usage advance to one of the one or more accounts associated with the second service class cases application of the second tariff data. In accordance with a further aspect of the invention there is provided a computer program product comprising a computer-readable medium having stored computer-readable program code for performing the steps of: receiving a request to provision a network usage advance, the request being associated with a subscriber identifier and a service class, wherein a network usage advance enables usage of a mobile telephone network by a prepaid mobile subscriber in advance of received payment, and wherein different categories of subscribers are associated with different service classes which correspond to different network usage advance services; obtaining service class data associated with the service class; and, transmitting an instruction to cause provision of the network usage advance to an account associated with the subscriber identifier in accordance with the service class data, wherein the service class data is usable by an intelligent network of the mobile telephone network to provision different network usage advance services based on the service class data. Further features provide for the computer-readable medium to be a non-transitory computer- readable medium and for the computer-readable program code to be executable by a processing circuit. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a schematic diagram which illustrates an exemplary system for provision of different network usage advance services to different categories of subscribers; Figure 2 is a schematic diagram which illustrates an example data structure which may be maintained by a charging system according to aspects of the present disclosure; Figure 3 is a schematic diagram which illustrates an example rating tariff table in which different types of accounts are associated with different tariffs according to aspects of the present disclosure; Figure 4 is a flow diagram which illustrates a method for provision of different network usage advance services to different categories of subscribers according to aspects of the present disclosure; Figure 5 is a swim-lane flow diagram which illustrates an example method for provision of different network usage advance services to different categories of subscribers in which service class data includes one or more account identifiers of accounts associated with a prepaid mobile subscriber; Figure 6 is a flow diagram which illustrates one example embodiment of how different tariffs are applied based on service class according to aspects of the present disclosure; Figure 7 is a block diagram which illustrates exemplary components which may be provided by a system for provision of different network usage advance services to different categories of subscribers; Figure 8 is a schematic diagram which illustrates an example user experience flow for a first type of subscriber in a first example category of subscribers in which a zero-fee network usage advance is provisioned; Figure 9 is a schematic diagram which illustrates an example user experience flow for a second type of subscriber in the first example category of subscribers in which a zero-fee network usage advance is provisioned; and Figure 10 illustrates an example of a computing device in which various aspects of the disclosure may be implemented. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS Aspects of the present disclosure enable the provision of different network usage advance services to different categories of subscribers. In particular, aspects of the present disclosure enable provision of a zero-fee credit service to a first set of subscriber categories and a fee-based credit service to a second set of subscriber categories. Aspects of the present disclosure may enable different categories of subscribers to be associated with different classes of network usage advance services. Service class associations may toggle or change dynamically in response to changes in, for example, credit data associated with the subscribers. A network usage advance enables usage of a mobile telephone network by a prepaid mobile subscriber in advance of received payment. In other words, an amount of airtime or a bundle may be provided to the subscriber in advance of payment for that airtime or bundle being received from the subscriber. Repayment may occur the next time the subscriber purchases or otherwise receives airtime or a bundle. Figure 1 is a schematic diagram which illustrates an exemplary system (100) for provision of different network usage advance services to different categories of subscribers. The system (100) may include a server computer (102) and an MNO platform (104). In some implementations, the server computer (102) may be provided by a third-party service provider platform (105) which provides network usage advance services to the MNO and its prepaid subscribers. The server computer (102) may implement the functionality of a service provider platform (105). In other implementations, the server computer may be built-in to or integrated with the MNO platform (without the need for a third-party service provider). The system (100) may further include communication devices (106), such as mobile handsets, associated with prepaid mobile subscribers who typically purchase airtime and mobile bundles to make use of a mobile telephone network (110) provided by the MNO. The MNO platform (104) and mobile telephone network (110) may collectively provide the infrastructure by way of which communication devices (106) can communicate with each other and the broader public switched telephone network (PSTN) and the Internet. The mobile telephone network may for example include one or more base station subsystems while the MNO platform may provide back-end systems such as a USSD/GPRS gateway, SMSC and associated components (VLR, HLR, etc.). The MNO platform (104) may include a prepaid gateway (113) an intelligent network (IN) (114) and a charging system (116). The prepaid gateway (113) may be configured, amongst other things, to detect requested usage of the mobile telephone network by a subscriber associated with a subscriber identifier, to capture information associated with the requested usage, such as the subscriber identifier, to transmit the captured information to the charging system (116), to receive permitted usage information from the charging system, to permit and monitor usage of the network in accordance with the permitted usage information and to report actual usage information to the charging system. The IN (114) may be implemented according to any suitable network architecture (e.g. that specified in the ITU-T Q.1200 series recommendations, or the like). The IN (114) may allow the MNO to provide various value-added services, such as network usage advance services, in addition to the standard telecom services such as PSTN, GSM, etc. The IN may include one or more network nodes on the service layer, distinct from the switching layer of the core network, as opposed to solutions based on intelligence in the core switches or equipment. The IN may be supported by the Signalling System #7 (SS7) protocol between network switching centres and other network nodes owned by network operators. The charging system (116) may be implemented as a node within the IN (114) that is responsible for collecting data on service usage and to generate reports thereon for billing, either on demand or automatically. In some implementations, the charging system may be an online charging system (OCS). The charging system (116) may be configured to authenticate and authorize network usage requests associated with subscriber identifiers received from the prepaid gateway (113), determine and transmit permitted usage information (such as a maximum call duration, etc.) to the prepaid gateway and to receive and charge the subscriber in accordance with actual usage information from the prepaid gateway (113). The charging system (116) may be implemented as or have access to a high-throughput database that maintains multiple accounts for each subscriber. The purpose, type and usage of the accounts may be defined by the MNO, in some cases upon request by the service provider. Each subscriber may be associated with one or more of: a main account, a dedicated account, a premium dedicated account, a loan tracking account (also referred to as a principal tracking account) and a fee tracking account. These accounts may be airtime accounts in that their value is denominated in airtime (which is typically equivalent to a value of currency, such as ZAR, USD, NGN, etc.). Each subscriber may further be associated with one or more data bundle, SMS bundle and voice minute bundle accounts, each of which may have its value denominated in bytes, a number of SMS messages and a number of minutes and seconds, respectively. Airtime top-ups may be provisioned to the main account and usage of the mobile telephone network (110) (such as making phone calls, sending SMS messages, transmitting and receiving data packets) by the subscriber may typically be charged against the main account. Network usage advances (such as airtime advances) may be provisioned to one of: the main account, the dedicated account or the premium dedicated account depending on a network usage advance service class (referred to herein as a service class) with which the subscriber is associated. Once the network usage advance has been provisioned, further use of the mobile telephone network may be charged against the account to which the network usage advance was provisioned until the balance thereof has been depleted. Data bundles purchased by the subscriber may be provisioned to a main data bundle account after which the transmission and reception of data packets by the subscriber’s communication device via the mobile telephone network may be recorded (or deducted from) the main data bundle account. Data bundle advances may be provisioned either to the main data bundle account or, in some implementations, to a dedicated data bundle account. Once provisioned, further transmission/reception of data packets may be recorded against the main data bundle account or the dedicated data bundle account, as the case may be, until the balance thereof has been depleted to zero. The same may apply, mutatis mutandis, for the SMS and voice minute bundle accounts. The tracking accounts may be used to keep track of amounts owed by subscribers in respect of network usage advances that the subscribers have requested and received. A non-zero balance of any one of the tracking accounts (be it loan tracking or fee tracking) may indicate an outstanding network usage advance. The value or amount of balance may indicate the amount owed by the subscriber in respect of the network usage advance. The accounts described herein may be implemented as fields of a high-throughput database. The fields may be associated with a subscriber identifier and can be updated or modified in real-time in response to network usage or top-up events (e.g. based on actual usage information received from the prepaid gateway). Different fields of the database may represent different accounts associated with a particular subscriber. In some implementations, the charging system (116) may include or be embodied by a data structure, such as a table maintained by relational database management system (RDBMS), having rows which are associated with subscribers (identified by way of a subscriber identifier, such as an MSISDN or other suitable identifier) and columns which are associated with account identifiers which identify and correspond to different types of accounts, such as each of the respective accounts described in the foregoing. An exemplary data structure (200) which may be maintained by the charging system (116) is illustrated in Figure 2 and includes: a subscriber identifier column (202); a main account column (204), with identifier ‘001’; a dedicated account column, with identifier ‘002’ (206); a premium dedicated account column, with identifier ‘003’ (208); a loan tracking account column, with identifier ‘004’ (210); and, a fee tracking account column, with identifier ‘005’ (212). Accounts associated with data, SMS and voice minute bundles are not illustrated for simplicity, but may be included as required by the relevant implementation. Rows (220, 222, 224, 226) of the table relate to individual entities (such as subscribers), identified by their relevant identifier (i.e. a subscriber identifier, e.g. ‘0000001’ etc.). The charging system (116) may have access to and/or maintain a rating tariff table (117) in which different types of accounts are associated with different tariffs. Figure 3 illustrates an example rating tariff table (117) in which a subset of the above accounts (i.e. those accounts configured for recording usage of the mobile telephone network by the subscriber, being ‘001’, ‘002’ and ‘003’) is each associated with a plurality of tariffs. Each of the plurality of tariffs is associated with a type of network usage and represents a chargeable rate for a particular unit of that type of network usage (e.g. phone calls in ZAR per minute, sending SMS messages in ZAR per message and transmitting/receiving data packets in ZAR per megabyte). The charging system and/or intelligent network may therefore be configured to apply different tariffs to different accounts. For example, as mentioned above, usage of the mobile telephone network by the subscriber may be charged against any one of the subscriber’s: main account, dedicated account, or premium dedicated account. However, the tariff applied to or charged for such usage may differ based on the account that is being charged. The charging system and/or intelligent network (or a node within the intelligent network or another part of the MNO platform) may be configured to apply first tariff data in respect of usage recorded against a premium dedicated account and second tariff data in respect of usage recorded against either a main account or the dedicated account. The intelligent network and/or charging system therefore maintains different accounts for the same subscriber which are configured for provision of different network usage advance services (e.g. by virtue of the different tariffs). Returning to Figure 1, the system (100) may include a triggering node (118) and a provisioning and recovery node (120). In some implementations, the triggering node (118) and provisioning and recovery node (120) may be provided by nodes of the IN (114). The triggering node (118) may be configured to monitor components or nodes of the MNO platform (104) and to detect predefined events. The triggering node (118) may for example be configured to monitor the charging system (116) for the occurrence of predefined events. In some implementations, the triggering node may be configured to monitor fields of the charging system. The predefined events may be low balance events, such as the value of a particular field falling below a predefined threshold. The triggering node (118) may be configured to derive a subscriber identifier and account identifier from an account at which the predefined event is detected. The triggering node (118) may be configured to output a trigger notification in response to detecting a predefined event. The triggering node (118) may be configured to include the subscriber identifier and the account identifier associated with the predefined event. The provisioning and recovery node (120) may be configured to provision airtime and/or mobile bundles to appropriate accounts of subscribers pursuant to top-up events and/or the provisioning of network usage advances. The provisioning and recovery node (120) may be configured to manipulate (e.g. access, change and/or update) fields of the charging system (116) so as to effect the provisioning of airtime and/or mobile bundles. In some implementations, the provisioning and recovery node (120) may be configured to query one or both of the loan tracking account and fee tracking account upon a subscriber top-up event and to divert some or all of the amount associated with the top up to one or both of the tracking accounts and/or relevant service provider account in order to effect partial or complete recovery of the outstanding network usage advance represented by these accounts. The MNO platform (104) may include a front-end API (126) by way of which the service provider platform and/or the server computer (102) may interface with components or nodes of the MNO platform, such as the charging system (116), triggering node (118) and provisioning and recovery node (120). The front-end API may interface with or provide access to an IN-integration component for aspects of the interface. The server computer (102) may be provided by any suitable computing device or devices. The server computer (102) may be configured to interface with the MNO platform (104). For example, the server computer may be configured to interface with the IN and/or charging system of the MNO platform via an appropriate IN integration component. Interfacing with the IN and/or charging system may enable the server computer to access and update selected data records stored therein. The service provider platform (105) may include a subscriber channel (121) via which the server computer (102) interfaces with the mobile telephone network (110) and in turn the communication devices (106). The server computer (102) may transmit and receive data and/or messages to and from the respective communication devices (106) via the subscriber channel (121) and the mobile telephone network (110). The server computer may for example be configured to receive network usage advance requests from the communication devices and to transmit notifications, prompts, confirmations, etc., relating to network usage advances to the communication devices. The service provider platform (105) may have access to a subscriber database (122) in which a record (124) associated with each subscriber is stored. Each subscriber record (124) may include and/or be associated with a subscriber identifier. In some implementations, each subscriber record includes one or more sets of credit data, such as a credit score, an outstanding network usage advance status, a network usage advance eligibility, a total credit limit and a service class assignment (i.e. an indication of a service class to which the subscriber has been assigned). The credit score may be an indication of credit worthiness or an indication of the risk associated with the subscriber. A high credit score may indicate low risk while a low credit score may indicate a high-risk subscriber. The total credit limit may be a total credit limit value denominated in a value of airtime. The credit limit value represents a value of a network usage advance for which a subscriber associated with the subscriber identifier is eligible in advance of payment. Each subscriber record (124) may additionally store identifiers of accounts which are associated with the subscriber. The subscriber record may for example include account identifiers for each of: the main account, dedicated account, premium dedicated account, loan tracking account and fee tracking account. Each subscriber record may additionally store an association or linking of each of the account identifiers to one or more service classes. For example, the premium dedicated account and loan tracking account may be associated with a first service class while the dedicated account, loan tracking account and fee tracking account may be associated with a second service class. The service provider platform (105), or the server computer (102), may have access to a service class assignment data structure (128) in which each service class is linked to corresponding service class data. The service class data may include, for each service class, one or more of: one or more account identifiers; tariff data; and, associated rules for provisioning in accordance with the service class. The rules may for example stipulate what amounts are to be recorded against each of the relevant accounts, which accounts to adjust, how to adjust them, which tariffs to apply, etc. The system (100) described above may implement a method for provision of different network usage advance services to different categories of subscribers. Figure 4 is a flow diagram which illustrates a method for provision of different network usage advance services to different categories of subscribers according to aspects of the present disclosure. In some embodiments the method may be carried out by a server computer (102), such as a server computer of a service provider platform (105) providing network usage advance services to subscribers of a mobile telephone network (110). In other embodiments, the method may be carried out by a computing device forming part of or otherwise connected to the MNO platform (104). The method may include assigning or associating (302) a prepaid mobile subscriber to or with a service class. The assignment or association may be based on credit data associated with the subscriber, for example, one or more of a credit score, network usage advance eligibility, total credit limit, outstanding network usage advance status or the like. In some implementations, for example, subscribers with low credit scores (e.g. equal to or less than a predetermined low value) and subscribers with high credit scores (e.g. equal to or greater than a predetermined high value) may be grouped into a single category (e.g. a “first category”) and may be assigned to or associated with a first service class. Subscribers with credit scores falling between the low and high scores (e.g. greater than the predetermined low value and less than the predetermined high value) of the subscribers in the first category may be grouped into a single category (e.g. a “second category”) and may be assigned to or associated with a second service class. In some cases, there may be other additional or alternative criteria that determine service class association, for example whether or not the subscriber is assigned to an outstanding network usage advance. For example, a subscriber having a credit score being equal to or greater than the predetermined high value is assigned to: the first service class if the subscriber is associated with an outstanding network usage advance status that indicates an outstanding network usage advance and if a total credit limit value with which the subscriber is associated is greater than zero; or, the second service class if the subscriber is associated with an outstanding network usage advance status that does not indicate an outstanding network usage advance or if a total credit limit value with which the subscriber is associated is equal to zero. In some cases, the service class may toggle based on the available credit limit value being equal to a predetermined threshold value. The predetermined threshold value may be determined on a per-subscriber basis using proprietary algorithms, e.g. so as to be based on the subscriber’s usual credit usage. Example groupings and associated user experience flows are described in greater detail towards the end of this disclosure. Of course, other groupings and associations or assignments may be made. Associating (302) the prepaid mobile subscriber with a service class may include storing the assignment or association in the subscriber record associated with the prepaid mobile subscriber. The association may be dynamic and updated in response to changes in the subscriber’s credit data, such as one or more of: credit score, credit limit, outstanding network usage advance status or the like. Thus, different categories of subscribers may be associated with different service classes which may in turn correspond to different network usage advance services. For example, subscribers having a high credit score, a high total credit limit and an outstanding network usage advance (with credit limit available) may be associated with the first service class. The same subscriber having no outstanding network usage advance may be associated with the second service class. Service class assignment may therefore change dynamically in response to events associated with the subscriber, such as the opening and closing of network usage advances. The method may include receiving (304) a request to provision a network usage advance. The request may be a subscriber originating request or an auto-generated request (e.g. generated automatically in response to detecting a low balance condition in an account associated with the subscriber). A subscriber originating request may be in response to transmitting a network usage advance offer to a mobile communication device associated with the subscriber identifier and receiving an acceptance of the offer from the communication device in the form of the request. The request may be associated with a subscriber identifier and a service class. In some implementations, the association may for example be by virtue of the request including the subscriber identifier, which in turn is associated with a subscriber record which in turn stores or is associated with a service class. In other implementations, the request may include one or both of the subscriber identifier and the service class. The method may include determining (306) a service class associated with the request. In some implementations, determining the service class may include querying the subscriber database using the subscriber identifier to identify the relevant subscriber record and extracting the service class stored therein. In other implementations, determining the service class may include extracting the service class from the request. In cases where a network usage advance offer is transmitted to a mobile communication device associated with the subscriber identifier, the method may include determining the service class prior to transmitting the usage advance offer. The offer may further include details relating to the service class, for example whether or not a fee will be charged and/or the applicable tariff. As mentioned above, in some cases, the service class and offer may be based on one or more of the credit score, total credit limit and outstanding network usage advance status associated with the subscriber. The method may include obtaining (308) service class data associated with the service class. The service class data is usable by an intelligent network of the mobile telephone network to provision different network usage advance services based on (or in accordance with) the service class data. In other words, the service class data may be used to control the type of network usage advance service. Service class data may include one or more account identifiers associated with the service class and/or tariff data associated with the service class. In some implementations, obtaining the service class data may include obtaining the service class data from a service class assignment data structure (128). In other implementations, obtaining the service class data may include obtaining the service class data from the subscriber record. The method may include transmitting (310) an instruction to cause provision of the network usage advance to an account associated with the subscriber identifier in accordance with the service class data. In some implementations, for example, the instruction includes the one or more account identifiers and causes provision of the network usage advance to an account associated with one of the one or more account identifiers and subscriber identifier. The different accounts may be associated with different tariffs such that provisioning the network usage advance to one account triggers application of a first tariff while provisioning the network usage advance to another account triggers application of another tariff. In other implementations, for example, the instruction includes tariff data and causes, firstly, provision of the network usage advance to an account associated with the subscriber identifier and, secondly, configuration of the intelligent network to apply a tariff in accordance with the tariff data. The instruction may be transmitted to the intelligent network or another component of the MNO platform, for example via the front-end API. The method may include updating (312) credit data, for example updating the total credit limit value by subtracting the value of the network usage advance and/or recording the existence of an outstanding network usage advance. The method may further include re-evaluating (314) the association based on the updated credit data. In some implementations, for some categories of subscribers, the service class with which the subscriber is associated may toggle based on the subscriber’s available total credit limit and/or outstanding network usage advance status. For example, a low risk subscriber having, e.g. a total credit limit of ZAR150, may be associated with a second service class while the available total credit limit is equal to ZAR150 (e.g. on account of no outstanding network usage advances) but may toggle to a first service class association in response to the available credit limit being less than ZAR150 (i.e. as soon as the subscriber requests and is provisioned a first network usage advance against the maximum available total credit limit, and hence is associated with an outstanding network usage advance). After provision of the network usage advance, the subscriber may be able to use the mobile telephone network using their communication device, with such usage being recorded against the appropriate account (i.e. the account to which the network usage advance is provisioned), and at the appropriate tariff, in accordance with the service class with which the subscriber is associated. In this manner, the appropriate network usage advance service is made available to the subscriber based on the service class with which the subscriber is associated (or based on the category into which the subscriber falls). For subscribers associated with the first service class, for example, the network usage advance may be provisioned to a premium dedicated account associated with the subscriber. This may include crediting the premium dedicated account with the value of the network usage advance (e.g. ZAR50) and also recording the value of the network usage advance against the loan tracking account such that the value of the outstanding network usage advance can be tracked. The intelligent network and/or charging system is configured to record usage of the network by the subscriber against the premium dedicated account at the first tariff associated with the first service class, which may be higher than a second tariff associated with a second service class. For subscribers associated with the second service class, for example, the network usage advance may be provisioned to a dedicated account associated with the subscriber. This may include crediting the dedicated account with the value of the network usage advance (e.g. ZAR50) and also recording the value of the network usage advance against the loan tracking account and further crediting a fee value (e.g. ZAR5) against the fee tracking account such that the value of the outstanding network usage advance can be tracked and a fee for the service can be charged to the subscriber. The intelligent network and/or charging system is configured to record usage of the network by the subscriber against the dedicated account at the second tariff associated with the second service class, which may be lower than the first tariff associated with the first service class. Any top-up or recharge event detected in association with an account (such as the main account) may trigger recovery of any outstanding network usage advances and fees (if appropriate) by applying the value associated with the top-up event towards recovery of positive balances stored in, for example, the loan tracking account and/or fee tracking account of the relevant subscriber. Figure 5 is a swim-lane flow diagram which illustrates an example method for provision of different network usage advance services to different categories of subscribers in which the service class data includes one or more account identifiers of accounts associated with a prepaid mobile subscriber. The respective swim-lanes are one example delineation of operations performed by the server computer (102) and intelligent network (114) (in some cases via the charging system) respectively. In other implementations, other delineations may be applied. Figure 5 illustrates in more detail an example embodiment of the method described above with reference to Figure 4, and any steps or operations described above with reference to Figure 4 that are not inconsistent with the embodiment described with reference to Figure 5 may apply mutatis mutandis to the method described with reference to Figure 5. A prepaid mobile subscriber may use his or her communication device (106) to transmit (402) a request for a network usage advance. The request may include or be associated with an amount or value (e.g. ZAR50) of the network usage advance and a subscriber identifier which uniquely identifies the subscriber. The subscriber and hence the subscriber identifier and/or the request may be associated with a service class (such as either a first service class being a zero-fee service class or a second service class being a fee-based service class). The request may be transmitted to the server computer (102) via the mobile telephone network (110) and subscriber channel (121). In some cases, the request may be in response to the server computer transmitting a network usage advance offer to a mobile communication device associated with the subscriber identifier and receiving an acceptance of the offer from the communication device in the form of the request. The server computer may determine particulars of the offer based on one or more of: a credit score, an outstanding network usage advance status, a network usage advance eligibility, a total credit limit and a service class assignment. This may include determining an amount or value of the network usage advance based on, for example, the credit score, a total credit limit and a service class assignment. For example, the server computer may offer a subscriber associated with the first service class and having a low credit score a network usage advance valued at double the total credit limit available to that subscriber. The server computer (102) may receive (404) the request and determine (406) the service class associated with the request. Determining the service class may include querying the subscriber database (122) using the subscriber identifier to identify the subscriber record and obtaining a service class stored in association with the subscriber record (e.g. by reading a service class field of the subscriber record). The server computer (102) may obtain (408) service class data associated with the service class. In some implementations the service class data is stored in the subscriber record and obtaining the service class data includes obtaining the service class data from the subscriber record. In other implementations, the server computer (102) has access to a service class assignment data structure (128) and obtaining service class data associated with the service class includes obtaining the service class data from the service class assignment data structure. In the illustrated embodiment, the service class data includes one or more account identifiers relevant to the provision of the network usage advance and optionally rules relating to the account identifiers and the provisioning. For example, service class data associated with the first service class may include account identifiers ‘003’ and ‘004’. Service class data associated with the second service class may include account identifiers ‘002’, ‘004’ ‘005’. The server computer (102) may transmit an instruction to cause provision of the network usage advance to an account associated with the subscriber identifier in accordance with the service class data. If (411) the subscriber is associated with a first service class, the server computer may transmit (410A) a first instruction message. If (411) the subscriber is associated with a second service class, the server computer may transmit (410B) a second instruction message. The instruction may include one or more of the subscriber identifier, the one or more account identifiers and associated operations to be performed against accounts associated with the account identifiers. The one or more operations may include or be based on the rules. For example, in the case of a request associated with the first service class, the instruction may include, for subscriber identifier 0000001, an instruction to provision the value of the network usage advance to account identifier 003 and record the value of the network usage advance against account identifier 004. In the case of a request associated with the second service class, the instruction may include, for subscriber 0000002, an instruction to provision the value of the network usage advance to account identifier 002, record the value of the network usage advance against account identifier 004 and record a value of a fee associated with the network usage advance against account 005. The fee may be a fixed value or a percentage of the network usage advance value. The instruction may be transmitted to the intelligent network (114) via the front-end API (126) of the MNO platform (104). The intelligent network (114) may receive the instruction and adjust the values or balances of the accounts accordingly. This may include adjusting balances in the accounts identified by way of the account identifiers in accordance with the instruction. For example, in the case of a first service class, the intelligent network may adjust (412A) the balance of account identifier 003 and account identifier 004 by ZAR50 each so as to provision the network usage advance and record the value of the outstanding network usage advance. In the case of a second service class, the intelligent network may adjust (412B) the balance of account identifiers 002, 004 and 005 so as to provision the network usage advance, record the value of the outstanding network usage advance and charge a fee for the provision of the network usage advance. As mentioned, at least one the accounts (e.g. ‘003’) associated with the first service class is configured for depletion in accordance with first tariff data while at least one of the accounts (e.g. ‘002’) associated with the second service class are configured for depletion in accordance with second tariff data. The intelligent network (114) may transmit (416) a response message to the server computer confirming the adjustments. The server computer may receive (418) the response message and transmit (420) a network usage advance confirmation message to the communication device (106). The communication device (106) may receive and display (422) the confirmation message. Once the network usage advance has been provisioned, usage of the mobile telephone network (110) by the subscriber using the communication device (106) may be permitted until the balance of the relevant account (be it the dedicated account, 002, or the premium dedicated account, 003) has been depleted. Depending on the account to which the network usage advance has been provisioned, different tariffs may be applied in respect of the usage. Figure 6 is a flow diagram which illustrates one example embodiment of how different tariffs are applied based on service class. The method may be carried out by the intelligent network (114) or the charging system (116). The method may include receiving (502) a network usage request (e.g. a request to make a phone call) associated with a subscriber identifier from a prepaid gateway. The method may include authenticating and authorizing (504) the request, including determining (506) permitted usage information. This may include querying account balances of accounts associated with the subscriber identifier and identifying an account with a positive balance. The method may include obtaining the account identifier and balance information of the account with a positive balance and using the account identifier to obtain tariff data associated with the relevant account (e.g. using a rating tariff table). The method may query accounts in a particular order (e.g. premium dedicated account, then dedicated account, then main account, or any other order) and identify the first account to have a positive balance. The method may include calculating permitted usage information (e.g. maximum duration of the call) using the balance information and tariff data and transmitting (508) the permitted usage information to the prepaid gateway. The method may include receiving (510) actual usage information (e.g. length of the phone call, once completed) from the prepaid gateway, determining (512) a value associated with the actual usage using the actual usage information and the tariff data associated with the identified account, and deducting (514) the value from the identified account. It should be appreciated that the account which is used to fund the usage (i.e. the identified account) determines the tariff data that is applied to the usage. Thus, by controlling to which account a network usage advance is provisioned, based on the service class with which the usage is associated, different tariffs can be applied for different service classes. Various components may be provided for implementing the methods described above with reference to Figures 4 and 5. Figure 7 is a block diagram which illustrates exemplary components which may be provided by a system for provision of different network usage advance services to different categories of subscribers. The system includes a server computer (102) which may form part of a service provider platform (105) or an MNO platform (104). The server computer (102) may include a processor (702) for executing the functions of components described below, which may be provided by hardware or by software units executing on the server computer (102). The software units may be stored in a memory component (704) and instructions may be provided to the processor (702) to carry out the functionality of the described components. The server computer (102) may include a request receiving component (706) arranged to receive a request to provision a network usage advance. The request may be associated with a subscriber identifier and a service class. In some cases, the request may be received from a communication device (106) of the subscriber via a mobile telephone network (110) and subscriber channel (121). In other cases, the request may be autogenerated, for example in response to detecting a low balance condition in an account associated with the subscriber. The server computer (102) may include a service class data obtaining component (708) arranged to obtain service class data associated with the service class. The service class data obtaining component may be configured to determine the service class with which the request/subscriber identifier is associated (e.g. by querying a subscriber database (122)). The service class data obtaining component (708) may be configured to query a service class assignment data structure (128) to obtain service class data associated with the service class. In some embodiments, the service class data includes account identifiers and instructions/rules for adjusting accounts in order to provision the network usage advance in accordance with the relevant service class. The server computer (102) may include an instruction transmitting component (710) configured to transmit an instruction to cause provision of the network usage advance to an account associated with the subscriber identifier in accordance with the service class data. The service class data is usable by an intelligent network of the mobile telephone network to provision different network usage advance services based on the service class data. For example, in the case of the service class data including account identifiers and instructions for adjusting account balances, the intelligent network may be able to provision the network usage advance to specific accounts being configured with specific tariffs (e.g. by virtue of a rating tariff table (117)) so as to provision a particular network usage advance service (e.g. being either a zero-fee service or a fee-based service). The system and method described herein may therefore enable subscriber category-based provision of network usage advance services, where different categories of subscribers are provisioned network usage advances having different service classes. This may enable a service provider and/or MNO providing network usage advance services to reach more subscribers by being able to tailor services to meet different subscribers’ requirements or needs. For example, some categories of subscribers (e.g. very high risk and very low risk subscribers) may prefer a zero-fee network usage advance service, whereas other categories of subscribers (e.g. intermediate risk subscribers) may prefer a fee-based network usage advanced service). The system and method described herein may therefore enable a Zero Fee Credit feature that can increase subscriber usage of the service (increased service penetration, credit usage maximization, user retention etc.), and boost generated revenues. A Zero Fee Credit service refers to a service that includes zero fees on the advanced amount but applies a Premium Tariff (higher depletion rates) on the usage of the advance (e.g. as described in the foregoing). The zero-fee credit feature (or zero-fee service) may target both subscribers with low Credit Limit (high risk), and also mid-high value subscribers (mid to high Credit Limit) who do not utilize their total lending power. The system and method described herein may comprise of or enable a new loan products scheme, intelligent user segmentation and service promotion. The systems and methods described herein enable a zero-fee airtime credit whereby the fee can be hidden in the inflated tariff of consuming the given airtime credit. Existing (prior art) network usage advance services may not be optimised (less loans, less advance amount, high risk, low credit usage) due to the applied loan fee, that has to be recovered on the next recharge. The system and method described herein enables a new service where network usage advances (e.g. Airtime Loans or Data Loans) do not include any fee (zero fee credit service), but a Premium Tariff plan (with higher depletion rates compared to the normal/base tariff plan) is applied to the advanced amount. This way the total loan amount is credited and is available on the subscriber’s account, the same amount is marked as outstanding (no exclusive or inclusive fee), but it is consumed/depleted with higher rates. For example: The system and method described herein may implement methodologies to offer the zero-fee service to two or more specific user segments. Using intelligent credit scoring methodology and applied models, the service provider may for example be able to identify the low value – high risk subscribers (e.g. subscribers with low Total Credit Limit – TCL), both adopters and non-adopters on this segment, and increase their lending power (e.g. double their TCL), allowing them to take more loans, with no extra fee applied but with the above mentioned Premium Tariff model (e.g. the first service using the first tariff data). The service provider may thus be able to maximize the value of this segment, by allowing users to spend more and also create a stronger retention. Low value subscribers who carry high risk, can now advance double the amount, making a significant impact on their spend on the network. Figure 8 illustrates an example user experience flow for this first type of subscriber in the first example category of subscribers in which a zero-fee network usage advance is provisioned but with a premium tariff being applied. Further, using intelligent data analytics and behaviour pattern analysis, the service provider may be able to identify high value (mid-high TCL) subscribers that do not maximize their Credit Limit to avoid extra fees. These subscribers who do not fully utilize their available TCL (e.g. by taking multiple lower value loans) can be approached with a Hybrid model. For example, by applying proprietary algorithms, the service provider can offer such subscribers the usual credit service (fee-based credit) up to their usual Credit Usage (but no more than that), and then the service provider can extend an overdraft to these subscribers (for the remaining credit limit), with zero fee, but with a higher depletion rate (Premium Tariff model) on the usage of the advanced amount. In this way, the service provider can maximize high-value subscriber’s Credit usage, and break the barriers by applying a Zero Fee Overdraft (but through a Premium Tariff model). Figure 9 illustrates an example user experience flow for this second type of subscriber in the first example category of subscribers in which a zero-fee network usage advance is provisioned but with a premium tariff being applied. The network usage advance services described herein may be accessible through various pull and push channels and marketing features. Subscribers qualifying for the zero-fee credit service (e.g. the specific subscribers’ segments as described above) may be notified using effective Marketing Campaigns to target and bring onboard these subscribers, promoting the Zero-fee Credit service (stating the applied Premium Tariff model). The system and method described herein may require integration with the IN (billing system) and a channel via which the subscriber can request the service (such as USSD / SMS / IVR / etc.). The system and method described herein may require that the IN provides a dedicated account (e.g. the premium dedicated account) that is configured to charge premium tariff on voice, SMS and data traffic. This account may be configured such that it cannot be used for item purchases, like VAS services, bundles, etc. The service provider may be configured to credit the premium dedicated account with the monetary amount requested by the subscriber (the loan amount) and, if loan tracking account(s) exist in the IN, it will also adjust those accounts accordingly. It might be the case, that different subscribers’ categories cannot be accommodated with the same premium dedicated account, due to the fact that different premium tariffs should be applied to different categories (e.g. different categories of mobile telephone network utilisation, such as voice calls, SMS messages, transmitting/receiving data packets, etc.). In this case, a different premium dedicated account may be defined per category. The service provide maintains the list of pairs (subscriber category <-> premium dedicated account ID) and credits the monetary amount to the account id that is assigned for the category. Figure 10 illustrates an example of a computing device (900) in which various aspects of the disclosure may be implemented. The computing device (900) may be embodied as any form of data processing device including a personal computing device (e.g. laptop or desktop computer), a server computer (which may be self-contained, physically distributed over a number of locations), a client computer, or a communication device, such as a mobile phone (e.g. cellular telephone), satellite phone, tablet computer, personal digital assistant or the like. Different embodiments of the computing device may dictate the inclusion or exclusion of various components or subsystems described below. The computing device (900) may be suitable for storing and executing computer program code. The various participants and elements in the previously described system diagrams may use any suitable number of subsystems or components of the computing device (900) to facilitate the functions described herein. The computing device (900) may include subsystems or components interconnected via a communication infrastructure (905) (for example, a communications bus, a network, etc.). The computing device (900) may include one or more processors (910) and at least one memory component in the form of computer-readable media. The one or more processors (910) may include one or more of: CPUs, graphical processing units (GPUs), microprocessors, field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs) and the like. In some configurations, a number of processors may be provided and may be arranged to carry out calculations simultaneously. In some implementations various subsystems or components of the computing device (900) may be distributed over a number of physical locations (e.g. in a distributed, cluster or cloud-based computing configuration) and appropriate software units may be arranged to manage and/or process data on behalf of remote devices. The memory components may include system memory (915), which may include read only memory (ROM) and random access memory (RAM). A basic input/output system (BIOS) may be stored in ROM. System software may be stored in the system memory (915) including operating system software. The memory components may also include secondary memory (920). The secondary memory (920) may include a fixed disk (921), such as a hard disk drive, and, optionally, one or more storage interfaces (922) for interfacing with storage components (923), such as removable storage components (e.g. magnetic tape, optical disk, flash memory drive, external hard drive, removable memory chip, etc.), network attached storage components (e.g. NAS drives), remote storage components (e.g. cloud-based storage) or the like. The computing device (900) may include an external communications interface (930) for operation of the computing device (900) in a networked environment enabling transfer of data between multiple computing devices (900) and/or the Internet. Data transferred via the external communications interface (930) may be in the form of signals, which may be electronic, electromagnetic, optical, radio, or other types of signal. The external communications interface (930) may enable communication of data between the computing device (900) and other computing devices including servers and external storage facilities. Web services may be accessible by and/or from the computing device (900) via the communications interface (930). The external communications interface (930) may be configured for connection to wireless communication channels (e.g., a cellular telephone network, wireless local area network (e.g. using Wi-Fi™), satellite-phone network, Satellite Internet Network, etc.) and may include an associated wireless transfer element, such as an antenna and associated circuitry. The external communications interface (930) may include a subscriber identity module (SIM) in the form of an integrated circuit that stores an international mobile subscriber identity and the related key used to identify and authenticate a subscriber using the computing device (900). One or more subscriber identity modules may be removable from or embedded in the computing device (900). The computer-readable media in the form of the various memory components may provide storage of computer-executable instructions, data structures, program modules, software units and other data. A computer program product may be provided by a computer-readable medium having stored computer-readable program code executable by the central processor (910). A computer program product may be provided by a non-transient or non-transitory computer- readable medium, or may be provided via a signal or other transient or transitory means via the communications interface (930). Interconnection via the communication infrastructure (905) allows the one or more processors (910) to communicate with each subsystem or component and to control the execution of instructions from the memory components, as well as the exchange of information between subsystems or components. Peripherals (such as printers, scanners, cameras, or the like) and input/output (I/O) devices (such as a mouse, touchpad, keyboard, microphone, touch-sensitive display, input buttons, speakers and the like) may couple to or be integrally formed with the computing device (900) either directly or via an I/O controller (935). One or more displays (945) (which may be touch-sensitive displays) may be coupled to or integrally formed with the computing device (900) via a display or video adapter (940). The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. Any of the steps, operations, components or processes described herein may be performed or implemented with one or more hardware or software units, alone or in combination with other devices. In one embodiment, a software unit is implemented with a computer program product comprising a non-transient or non-transitory computer-readable medium containing computer program code, which can be executed by a processor for performing any or all of the steps, operations, or processes described. Software units or functions described in this application may be implemented as computer program code using any suitable computer language such as, for example, Java™, C++, or Perl™ using, for example, conventional or object-oriented techniques. The computer program code may be stored as a series of instructions, or commands on a non- transitory computer-readable medium, such as a random access memory (RAM), a read-only memory (ROM), a magnetic medium such as a hard-drive, or an optical medium such as a CD- ROM. Any such computer-readable medium may also reside on or within a single computational apparatus, and may be present on or within different computational apparatuses within a system or network. Flowchart illustrations and block diagrams of methods, systems, and computer program products according to embodiments are used herein. Each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, may provide functions which may be implemented by computer readable program instructions. In some alternative implementations, the functions identified by the blocks may take place in a different order to that shown in the flowchart illustrations. Some portions of this description describe the embodiments of the invention in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations, such as accompanying flow diagrams, are commonly used by those skilled in the data processing arts to convey the substance of their work effectively to others skilled in the art. These operations, while described functionally, computationally, or logically, are understood to be implemented by computer programs or equivalent electrical circuits, microcode, or the like. The described operations may be embodied in software, firmware, hardware, or any combinations thereof. The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention set forth in any accompanying claims. Finally, throughout the specification and any accompanying claims, unless the context requires otherwise, the word ‘comprise’ or variations such as ‘comprises’ or ‘comprising’ will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.