A system for client initiated authentication comprises a user agent client and a user agent server. The user agent client is operable to communicate a session initiation protocol request. The session initiation protocol request comprises an authenticate header and a require header that comprises a server authentication tag. The user agent server is operable to receive the session initiation protocol request. The user agent server is further operable to communicate a session initiation protocol response in response to the session initiation protocol request. The session initiation protocol response comprises an authorization header having a credential of the user agent server.