Solutions are described that enable a user equipment (UE) for a communication system always to derive required identifiers using an international mobile subscriber identity (IMSI) even when the UE has only a subscriber identity module. One solution is based on a table, or list, that is maintained in the UE. Another solution is based on always taking a predetermined number of digits of the IMSI. Yet another solution is based on try-and-fail signaling. In consequence, the UE does not always need to know how many digits of the IMSI are used to indicate a mobile network code, which in some situations is impossible to know anyway.