A proxy server causes an authentication authority to authenticate a client in response to a first Session Initiation Protocol (SIP) request of the client on a connection. It does not cause the client to be authenticated in response subsequent requests on the connection as long as the underlying connection is not broken, the subsequent requests are on behalf of the same client, the client has not been removed from the system, the client's password has not changed, a “safety net” timer has not expired, or any other policy that the server chooses to enforce. This eliminates the overhead of constant re-authentication in response to each SIP request.