The phone number used by a caller accessing a telecommunication service is derived. For example, this phone number may be used to look up in a first database the name of the user registered with the service, and in a second database the name of the subscriber assigned the phone number. If the two names correspond, the caller is provided with the telecommunication service. Otherwise, a fraud has been detected and various consequences may be visited upon the caller or the user. Some of these consequences include denying use of the service, charging the caller extra for use of the service, etc.