Abstract: Process for controlling the access of a user to a service provided in a data network, to protect user data stored in a data base of the service from unauthorized access, the method comprising: (a) inputting, in a VoIP session, a voice sample of the user at a user data terminal which is at least temporarily connected to the data network, (b) processing, in a first processing step, the user's voice sample using a dedicated client implemented at the user data terminal, to obtain a pre-processed voice sample or a current voice profile of the user, (c) further processing, in a second processing step, the pre-processed voice sample or the current voice profile, including a comparison step of the current voice profile with an initial voice profile stored in a data base, and (d) outputting an access control signal for granting or rejecting access to the service, taking the result of the comparison step into account.