Abstract: A method and equipment for using two SIM cards with the same MSISDN number in a mobile communication system. To at least one subscriber identifier are allocated at least two identity modules (SIM), of which only one at a time can be registered as active. In connection with location updating it is checked whether the location updating relates to a subscriber identifier to which at least two identity modules (SIM) have been allocated. If yes, it is checked whether the identity module (SIM) concerned is at that particular moment registered as passive, and if yes, it is activated 3and the identity module (SIM) earlier registered as active is deactivated. If the identity module (SIM) earlier registered as active is involved in an on-going call while location updating is being performed using an identity module (SIM) earlier registered as passive, the location updating is rejected or delayed until the on-going call has been terminated.