an MS can have several MSISDNs depending on the SIM. The subscriber identity cannot be derived from the MSISDN unless the association of IMSI and MSISDN as stored in the HLR is known. In addition to this a subscriber can hold several MSISDNs for selection of different services. Each MSISDN of a subscriber is reserved for a specific service (voice, data, fax, etc.). In order to realise this service, service specific resources need to be activated, which are done automatically during the setup of connection. The MSISDN categories have the following structure: Second Generation Cellular Networks Country Code (CC): up to 3 decimal places; National Destination Code (NDC): typically 2-3 decimal places; Subscriber Number (SN): maximum 10 decimal places. Thus, MSISDN = CC + NDC + SN. Mobile Station Roaming Number (MSRN) The Mobile Station Roaming Number (MSRN) is a temporary location dependent ISDN number. It is assigned by a locally responsible VLR to each MS in its area. Calls are routed by the MS using the MSRN. On request, the MSRN is passed from the HLR to the GMSC. The MSRN has the same structure as the MSISDN: Country Code (CC) of the visited network; National Destination Code (NDC) of the visited network; Subscriber Number (SN) in the current mobile network. The components CC and NDC are determined by the visited network and depend on the current location. The SN is assigned by the current VLR and is unique within the mobile network. The assignment of an MSRN is done in such a way that the currently responsible switching node MSC in the visited network (CC + NDC) can be determined from the subscriber number, which allows routing decisions to be made. The MSRN can be assigned in two ways by the VLR: either at each registration when the MS enters a new Location Area (LA) or each time when the HLR requests it for setting up a connection for the incoming calls to the mobile station. In the first case, the MSRN is also passed on from the VLR to the HLR, where it is stored for routing. In the case of an incoming call, 