When a subscriber wants to call a mobile station (MS), the network nee ds to know the location of the called MS in order to route the call to the appropriate Mobile services Switching Centre (MSC) (see GSM 03.12 on location registration). This Global System for Mobile communicatio ns Technical Specification (GTS) describes how the routeing process ca n be performed, and considers the effect on the routeing process of th e capabilities of the exchanges involved. This GTS assumes that the Mobile Station ISDN Number includes a specif ic National Destination Code which identifies the Home PLMN (HPLMN) of the called MS (referred to below as the HPLMN). If the numbering pl an for MSs is fully integrated into the fixed network numbering plan, the method of routeing may be different. In the diagrams in clause 6, the originating local exchange is shown a s being part of an ISDN. The same routeing principles apply if the ori ginating local exchange is part of a PSTN. Short Messages are routed in the same way as circuit-switched calls; t he SMS Gateway MSC must be able to interrogate the HLR of the MS for w hich the short message is intended, in order to route the short messag e to the visited MSC.