1.0 1xx Responses
These responses are classed as "Provisional Responses" and indicate that the server is performing some sort of action and does not yet have a definitive response based on the request it has received.
1.1 100 Trying
The server has received the request and is taking action based on the nature of the request.
1.2 180 Ringing
The phone receiving a call request is trying to alert the user of the incoming call.
1.3 183 Session Progress
Conveys information about the progress of a call.
2.0 2xx Responses
These responses are classed as "Successful Responses".
2.1 200 OK
The request has succeeded.
3.0 3xx Responses
These responses are classed as "Redirection Responses", and convey information about a callee's new location or any alternative services.
3.1 301 Moved Permanently
The callee is no longer found at the dialled address and should be contacted at the new specified address (which is given in the response). Future call attempts should go to the new address.
3.2 302 Moved Temporarily
The callee is temporarily not available at the dialled address and should be contacted at the new specified address (which is given in the response). Future call attempts should still go to the original address.
4.0 4xx Responses
These responses are classed as "Failure Responses". They indicate a failure of a particular client or server to respond to a particular request.
4.1 400 Bad Request
The request could not be understood by the server, most probably due to the request having bad syntax in its structure.
4.2 401 Unauthorized
The request requires authentication. This normally means that username and password credentials need to be supplied.
4.3 403 Forbidden
The request was understood by the server, but validation of the request has been denied. It is unlikely trying the same request again will succeed. The server may give a reason why it denied the request, but this is not compulsory.
4.4 404 Not Found
The callee does not exist at the called number. You will also get this message if the callee's client is not registered with their server, and has not been registered for at least five to ten minutes.
4.5 405 Method Not Allowed
The request was understood but it is not being allowed. The server should give a list of request-types that it will accept.
4.6 407 Proxy Authentication Required
Similar to code 401, this means that username and password credentials MUST be supplied.
4.7 408 Request Timeout
The server could not process the request in a suitable amount of time.
4.8 410 Gone
The callee is not available at the dialled number AND the server has no idea how to contact them.
4.9 415 Unsupported Media Type
This normally means that the caller and callee have failed to negotiate a codec to use for the phone call (i.e. neither party supports a common codec). For VoIP-PSTN calls, the codec negotiation has failed between the VoIP phone and the VoIP gateway server. This situation can only be resolved by determining the supported codecs at either end of the call and figuring out a way to have both ends use a common choice. It may require using a different VoIP phone which has a larger range of codec support.
4.10 480 Temporarily Unavailable
The callee was successfully contacted but is not available for whatever reason (e.g. they may have enabled "Do Not Disturb" on their phone). The only solution is to try again in five minutes or so.
4.11 481 Call/Transaction Does Not Exist
The request was received by the callee but the request failed to match any existing or previous transaction and therefore the callee cannot process it.
4.12 482 Loop Detected
The server has received a request with the same destination it just received the request from.
4.12 483 Too Many Hops
The server received a request with the "Max-Forwards" field set to zero. This means the request went through too many points to get to the server. Some servers give this message if the caller is sending the request from a private IP address (i.e. STUN and uPnP is off on the client). The request may be tried again in a few seconds if desired.
4.13 484 Address Incomplete
The server received a request with an incomplete destination address. The address must meet the format "[number]@[sipserver]:[port]".
4.14 486 Busy Here
The callee was contacted successfully but cannot or will not take the call at this point in time.
4.15 487 Request Terminated
The sent request was terminated with a BYE or CANCEL from either the server or callee. This response can be sent in situations where a call is set to ring on many phones and gets picked up on one phone. All phones that do not pick up the call will receive this response.
4.16 488 Not Acceptable Here
The callee received the request successfully but rejected the session description, possibly due to bad choice of codec, unacceptable IP address and port for sending audio to, or even bad formatting of the session description itself. It is possible this request may succeed if addressed elsewhere.
4.17 491 Request Pending
The callee has already receive the same request and is currently trying to process it.
5.00 5xx Responses
These responses are classed as "Failure Responses" like the 4xx class, but these are only generated by a server.
5.01 500 Server Internal Error
The server encountered a problem that has prevented it from fulfilling the request. The request can be sent again after a few seconds.
5.01 501 Not Implemented
The server does not have the means necessary to fulfill the request.
5.02 503 Service Unavailable
The server is either overloaded or under maintenance and temporarily cannot process the request. Servers under this condition do not necessarily have to respond with this message at all, they can just ignore/deny the request and drop the call. If the server does not say when the request can be retried, the client should act as if it had received a "500 Server Internal Error" response.
5.03 505 Version Not Supported
The server does not support the SIP protocol version used in the request.
6.00 6xx Responses
These responses are classed as "Global Failures", and usually indicate that a party in the call request has rejected the current call request, and may reject all such requests in the future unless it indicates otherwise.
6.01 603 Declined
The callee was contacted successfully but they do not wish to or cannot participate. This response is usually returned when the callee knows that the request cannot be diverted elsewhere.
6.02 606 Not Acceptable
The callee received the request successfully but rejected the session description, possibly due to bad choice of codec, unacceptable IP address and port for sending audio to, or even bad formatting of the session description itself. All requests sent to this destination using the same session description are likely to get the same response.