A method and apparatus for the control of a subscriber identity module (SIM) in a data communication system, preferably a mobile communication system. The data communication system includes first and second subscriber registers (HLR1, HLR2) for maintaining subscriber records defining a subscriber identity module registry, a short message transmission system (SMSC) for transmission of messages in the communication system, and a mobile station (MS) connected to the subscriber identity module (SIM) for use by a subscriber in effecting mobile communications through the data communication system. The subscriber identity module (SIM) stores therein data comprising a first subscriber code (IMSI1) and an encrypted code key (K_i) corresponding to a first subscription for the mobile station subscriber and associated with the subscriber identity module A record of data corresponding to the first subscription is created in the first subscriber register (HLR1) when the first subscription is opened, the record comprising a first subscription specific call number (MSISDNx), the encryption code key (K_i) and a subscriber identity code (IMSI1) for the mobile station subscriber. A second subscription for the mobile station subscriber and associated with the subscriber identity module (SIM) is opened; a record corresponding to the opened second subscription and comprising a second subscription-specific call number (MSISDN), a second subscriber identity code (IMSI1) and the encryption key (K_i) is created in the second subscriber register (HLR2); a message (SMS) is sent through the communication system directed to the first subscription and instructing that the data stored in the subscriber identity module be changed from data corresponding to the first subscription to data corresponding to the second subscription.