A short list of the more common SIP 400 Error codes.
The request requires user authentication.
The server understood the request, but is refusing to fulfil it. (Bad password, no account, etc)
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.
409 “Conflict" back from 10.xxx.xxx.xxx
Someone or something is already registered using same account; The server does not allow more than one registration at the same time using same account.
The user existed once, but is not available here anymore. i.e. the PC \ Extension has gone offline.
412 Conditional Request Failed
The given precondition has not been met. (The session requirements could not be met. i.e. protocol, codec, ports etc. )
415 Unsupported Media Type
Request body in a format not supported. The Codec’s offered in the session are not supported, i.e. g729, g711 etc..
483 Too Many Hops
Max-Forwards header has reached the value '0', during the session setup too many connection devices have been hit, i.e. Routers, switches, etc.. Are there small switches used to split a connection out to PC, Printer, phone, etc..
487 Request Terminated
Request has terminated by bye or cancel. One of the parties has hung up prior to the call connection completing, or a condition caused one of the SIPs to Terminate the call.