Tunnel Redirect
You can use tunnel redirect when requests are routed to a server behind a SIP NAT that sends redirect responses with addresses that should not be modified by the SIP NAT function. For example, a provider might wish to redirect certain calls (like 911) to a gateway that is local to a the UA that sent the request. Since the gateway address is local to the realm of the UA, it should not be modified by the SIP NAT of the server’s realm. Note that the server must have a session agent configured with the redirect-action field set to the proxy option in order to cause the redirect response to be sent back to the UA.