SIP/2.0 404 Not Found
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404 Not Found:
Sent from the proxy when either the intended party does not exist (wrong number dialed) or the user is not registered on that proxy. This message might be sent from the gateway if it's caused by dialing a wrong number.
This is also sent from the proxy in response to INVITE request if device is not registered to that proxy. You can tell if this is the case because the proxy does not authenticate the Invite request with a "407 Authentication Required".