Abstract: An operating system for a mobile phone (1) includes a mobile phone having storage capacity adapted to the operating system, such as RAM, ROM, PROM or E-PROM, and a user identifier card provided by the operator, such as a SIM card (8). The data carried by the user identifier card supplied by the operator can be fed into the storage space of the mobile phone (1) from outside the mobile phone, with the user identifiers (20-23) of more than one operator being fed to the same mobile phone.