Below is a list of selected response codes that may be sent from a SIP server. A full list can be found here.
1xx – Provisional responses
- 100 Trying: An extended search that is being performed may take a significant time, so a forking proxy must send a 100 Trying response.
4xx – Client failure responses
- 403 Forbidden: The server understood the request but is refusing to fulfill it.
- 404 Not Found: The server has definitive information that the user does not exist at the domain specified in the Request-URI. This status is also returned if the domain in the Request-URI does not match any of the domains handled by the recipient of the request.
- 408 Request Timeout: The server could not produce a response within a given time, for example because it could not determine the user's location. The client may repeat the request without modifications at any later time.