Abstract: An apparatus and a method for providing at least two identities to a subscriber unit of a mobile telephone system. A subscriber identity module (SIM) is provided which is inserted into the subscriber unit for selectively activating an identity. The module may be activated through a keypad on the subscriber unit allowing a subscriber to enter a code for selecting a specific identity, or it may be a card having a separate program circuit on either end so that when inserted into the unit with its first end, a first identity is activated, and when inserted into the unit with its second end, a second identity is activated. Each selected identity may be allocated a separate directory number or all identities may be allocated a single number.

Abstract: Personalization of an active so-called SIM card for a mobile telephone system of the GSM type is effected in a place connected to the central computer of the system via a communication network. The card identity IMSI and card authentication key Ki are transferred in line-encrypted form to terminal equipment in that place, where the card is inserted in a reader. After line decryption, the result thereof is transferred to the card in a manner to prevent unauthorized listening-in. This can be done in a safety box or by double encryption of IMSI and Ki, the decryption thereof being performed within the card by means of a unique card key Ksim.