When a subscriber wants to call a mobile station (MS), the network needs to know the location of the called MS in order to route the call to the appropriate Mobile Services Switching Centre (MSC) - see TS 03.12
on location registration. This TS describes how the routeing process can be performed, and considers the effect on the routeing process of the capabilities of the exchanges involved.
This TS assumes that the Mobile Station ISDN Number includes a specific National Destination Code which identifies the Home PLMN (HPLMN) of the called MS (referred to below as 'the HPLMN'). If the numbering plan for MSs is fully integrated into the fixed network numbering plan, the method of routeing may be different.
In the diagrams in section 6, the originating local exchange is shown as being part of an ISDN. The same routeing principles apply if the originating local exchange is part of a PSTN.
Short Messages are routed in the same way as circuit-switched calls; the SMS Gateway MSC must be able to interrogate the HLR of the MS for which the short message is intended, in order to route the short message to the visited MSC.