Abstract: In a telecommunications system such as a global mobile telephone network in which each subscriber unit includes a Subscriber Identity Module (SIM card), each SIM card has fixed memory locations (22), to which data can be addressed over the air. Some of the locations (22) can not be overwritten from the subscriber unit but can be accessed therefrom on the entry of short simple codes, each associated with one of the locations. Further fixed memory locations (24) can be read over the air only when the subscriber enters a personal identification number. Locking control files (27, 28) are used to control read/write access to the locations (22, 24) respectively.