An identity module (SIM 101) is activated in the following manner. A customization center (106) stores an initial identity code (IMSI) in the identity module (101) and communicates the initial identity code to a network management system (103, 104). A distributor (107, 108) assigns a communication address (MSISDN) to the identity module and (101) communicates the communication address together with the initial identity code to the network management system (103, 104). The identity module (101) contacts the network management system (103, 104) via a communication device (mobile phone), whereby the network management system (103, 104) receives the initial identity code together with the communication address. The network management system (103, 104) recognizes the initial identity code together with the communication address and, in response, assigns an effective identity code to the identity module (101). Subsequently, the network management system (103, 104) sends a message to the communication device (mobile phone). This message (a special SMS) comprises the effective identity code. The identity module (101) receives the message via the communication device, extracts the effective identity code from the message and stores the effective identity code for future use.