MoU have imposed two constraints for the charging of optimally routed calls:

• No subscriber shall pay more for a call which has been optimally routed than he would do under the present routeing scheme described in GSM 03.04 [1] in the clauses describing the call cases where the GMSC is in the same PLMN as the HLR.
• At least for the first phase of Support of Optimal Routeing, the charge for one leg of a call shall be paid for entirely by one subscriber.

These constraints mean that the direct route for a call cannot always be used. For example, if the calling mobile subscriber (the A subscriber) is in Germany, and the B subscriber's HPLMN is in Switzerland but he has roamed to Finland, the charge payable by the A subscriber to route the call by the direct route to Finland would be greater than the charge payable to route the call to HPLMNB, so the HPLMN route must be used. In the first phase of Support of Optimal Routeing, it cannot be assumed that a GMSC is able to calculate the charge payable for the direct route and the charge payable for the HPLMN leg. The MoU requirements can be met by applying more stringent (but simpler) criteria for deciding whether the direct route may be used:

• If the country code of the destination exchange and the country code of the GMSC are the same, then the direct route may be used.
• Otherwise, for a call leg which is chargeable to the A subscriber, if the country code of the destination exchange and the country code of HPLMNB are the same, then the direct route may be used.
• Otherwise, the HPLMN route shall be used.

In certain cases, the second criterion above (equality of country codes for the HPLMN and the destination exchange) may not be enough to determine equality of the charges payable for the direct route and the HPLMN route. In these cases, analysis of the national destination code as well as the country code is required; however the principle is still that if the two numbers are the same to the depth of analysis required then the direct route may be used. For optimal routeing of late call forwarding, the constraints are satisfied if the following criteria are applied:

• if the country code of the forwarded-to exchange and the country code of the GMSC are the same, then the forwarded call may be routed directly from the GMSC to the forwarded-to exchange;
• otherwise, if the country code of the forwarded-to exchange and the country code of HPLMNB are the same, then the forwarded call may be routed directly from the GMSC to the forwarded-to exchange;
• otherwise, if the country code of the forwarded-to exchange and the country code of VPLMNB are the same, then the forwarded call may be routed directly from the GMSC to the forwarded-to exchange;
• otherwise the forwarded call shall be routed through VPLMNB.

The functional behaviour of VMSCA is specified in TS 23.018. The only functions specific to optimal routeing are:

• Routeing the call to the associated GMSC function if the gsmSCF indicates that the call is eligible for optimal routeing;
• The transfer of the destination address, if it is received in the indication of the answer event, to the call data record, to allow the correct charge for the call to be made. This function is required only if VMSCA supports optimal routeing of mobile-to-mobile calls.

The functional behaviour of VLRA is specified in TS 23.018.

When a call is being forwarded the forwarding leg is controlled by a process MT_CF_MSC which runs in the forwarding MSC (VMSC or GMSC). If CAMEL processing of the forwarding leg indicates that Basic Optimal Routeing of the forwarding leg should be attempted, the forwarding MSC may use the services of an associated GMSC for the forwarding leg, i.e. the associated GMSC requests routeing information from HLRC. In this case, the forwarding leg is processed in the same way as a mobile-originated call from mobile subscriber B. The functional behaviour of a GMSC is specified in TS 23.018. The procedures specific to Support of Optimal Routeing are specified in this clause.