Abstract: The present invention concerns a method and apparatus for restricting the right of use of a mobile station, for example, to a communications network controlled by a certain operator. The mobile station is locked so that it can be used only with a SIM module (21) having a certain code (IMSI, GID1, GID2), and the lock can be opened with a lock open password (C.sub.1) given by the network operator, the password being compared in the mobile station with a comparative password (C.sub.2). In the method and apparatus according to the invention the lock open comparative password (C.sub.2) is calculated in the mobile station by an algorithm (X, Y) stored in the mobile station, input data for the algorithm being preferably an equipment table (T.sub.1), a mobile equipment identity code (IMEI), and an operator specific code N, all stored in the mobile station. Correspondingly, the operator calculates the lock open password (C.sub.1) with a device that preferably includes a protection module.