Abstract: A method of unlocking a mobile terminal after a period of use and ensuring that the first operator who sold the terminal receives adequate payment prior to the use thereof. The identity of the terminal is saved and a security algorithm, which is not known to the operator, is implemented in a security module which is introduced into a server that is managed by the operator. After the period of use, the security module determines a secret key which is obtained by applying the terminal identity transmitted thereby and an operator code to the security algorithm, following a request transmitted by the service entity. The determined secret key, which is encrypted at the terminal, is transmitted over the operator's radiocommunications network and decrypted in the mobile terminal so that it can be compared to the saved secret key in order for the mobile terminal to be unlocked when the compared secret keys are identical.