A technique for authenticating a user to a server using SIP messages includes forwarding an SIP request from the user agent to the server. The server then forwards a request for authentication to the user agent in response to the invite request, the request for authentication including information that the authentication will be performed using a UMTS AKA mechanism. The user agent then forwards and authentication response to the server in accordance with the UMTS AKA mechanism and the server then performs the appropriate actions to perform an invoked SIP procedure in response to the SIP request. The SIP request may include any standardized SIP request including an SIP INVITE request or an SIP REGISTER request.