A mutli-system subscriber identification module

A subscriber identification module (SIM) for storing user-identities and other user specific information and thereby enabling access to the service of the radio system. One of the user identities is a Temporary Mobile Subscriber Identity (TMSI) used for user anonymity purposes by utilizing the TMSI as an alias for the actual user identity in order to avoid sending the actual user identity in the clear on the radio path. A part of the TMSI is a random number which cannot be used to identify the user/terminal without mapping information to the actual user identity, which is found in the telecommunications network entity that allocated the random number. SIM roaming from one radio system to another is possible by utilizing predetermined bits of TMSI to differentiate between different TMSI's of different systems by indicating in a uniform manner in all mobile radio systems the mobile radio system which has allocated the TMSI. The remaining bit locations of the TMSI provided in a mobile radio system-specific format. During an inter-system SIM roaming, the old TMSI will be copied from SIM and forwarded to a new mobile radio system. The new system will use the predetermined bits of the TMSI to identify the old system in order to request the actual user identity.