A method for user authentication in a telecommunication network includes an authentication server with an authentication service and comprises an initial identity check of a upon a user request. The method includes steps performed by the authentication server, in which steps the authentication server receives a request with user behavior data for further authentication of a user, stores the user behavior data, matches the initial identity with previous information of the user by using an algorithm, performs further checking of the identity of the user if the initial identity of the user and the previous information of the user did not match in a predetermined manner, and reports the result of the further authentication as a reply to the request. A system performs the steps of the method.