The invention relates to a method for subsequently implementing a SIM functionality, with the help of which a mobile telephone (1) is enabled to use a mobile radio network, in a security module (3). Within the context of the method according to the invention the SIM functionality is realized in the form of an application, of which at least a first part is loaded into the security module (3). Furthermore, personalizing data, which are required for a use of the mobile radio network by the mobile telephone (1), are transmitted from a provider (2) to the security module (3) in encrypted form on the direct or indirect way. The encrypted personalizing data are decrypted by the security module (3) by means of a secret key of a user stored in the security module (3). The security module (3) is personalized by means of the decrypted personalizing data.