Abstract: A method and system for determining that a SIM and a SIP client are co-located in the same ME. A SIP server, an SLR, and the ME are communicatively interconnected in the system. The SIM has an IMSI and is arranged in the ME. In the method and system, a security module, SLM-SM, included in SIM, encrypts, using a key, a variable derived from the IMSI, and provides a first password parameter as a result of the encrypting. Further, a SIP client location manager, SLM, included in the ME, receives the ME security code; performs an XOR operation of the ME security code and the identity of the ME, and provides a second password parameter as a result of the XOR operation.

Abstract: A method for serving visitor subscribers in a mobile communication system has been disclosed. The method, which is performed by a virtual visitor subsystem, VSS, included in the mobile communication system, comprises the following steps: detecting a roaming event on an MSISDN of a mobile station, MS, operating in the mobile communication system; determining a mobile network operator in the mobile communication system, having an HPLMN which covers a geographic area of the MS, to be a visitor mobile network operator of the MS; receiving a visitor IMSI from the visitor mobile network operator; assigning the visitor IMSI to the MSISDN of the MS; transmitting the visitor IMSI to the MS; intercepting an authentication initiation towards the visitor mobile network operator; authenticating the MS towards a home mobile network operator of the MS; and activating a visitor subscription identified by the visitor IMSI based on an authentication response received from the home mobile network operator.