03.043GPPSignalling Requirements Relating to Routing of Calls to Mobile SubscribersTS
If the calling subscriber is served by an exchange which is part of the HPLMN, the configuration is as shown in figure 1. The originating MSC is able to interrogate the HLR for an MSRN, and the call is routed within the PLMN. If the called MS has roamed to another PLMN the originating MSC routes the call to the visited PLMN (VPLMN) as shown in figure 2.
Figure 1: Call from a subscriber in the HPLMN of the called MS
Figure 2: Call from a subscriber in the HPLMN of the called MS; called MS roamed to another PLMN