An MSC which receives an IAM from an originating exchange may react in three different ways:

• It acts as a transit exchange, i.e. it relays the IAM to a destination exchange determined by analysis of the called party address, and thereafter relays other ISUP signalling between the originating and destination exchange until the connection is released. This behaviour is not specific to UMTS or GSM.
• It acts as a terminating exchange, i.e. it attempts to connect the call to an MS currently registered in the service area of the MSC.
• It acts as a GMSC, i.e. it interrogates an HLR for information to route the call. If the HLR returns routeing information, the MSC uses the routeing information from the HLR to construct an IAM, which it sends to a destination exchange determined by analysis of the routeing information from the HLR.

The method which the MSC uses to determine how to handle the IAM is described in TS 23.018. However, the number analysis required to derive the address of an HLR in a different PLMN from the MSC is much more extensive than that required to derive the address of an HLR in the same PLMN as the MSC - the MSC needs to be able to recognise the combination of country code and national destination code for every subscriber of every PLMN to which calls are to be optimally routed. In order to avoid the need to maintain the tables required to support this extensive number analysis, and the run-time processing load of performing the number analysis, optimal routeing of mobile-to-mobile calls is handled as a mobile originating CAMEL service; the gsmSCF analyses the destination address (after possible modification of the number by other MO CAMEL services) to determine whether the call is eligible for optimal routeing. If the call is eligible for optimal routeing, the gsmSCF indicates this to VMSCA, which routes the call to an associated GMSC function, which in turn sends a request for routeing information to the appropriate HLR. When a GMSC has interrogated an HLR and received an MSRN, the GMSC may need to route the call to the HPLMN of the called subscriber. If the call is routed through an MSC which has the capability to analyse an address to derive an HLR address, a method must be provided to prevent the transit MSC from performing a further interrogation of the HLR, using the MSRN as an MSISDN. The method used to prevent this further interrogation is a matter for the PLMN operator.