The SIP 500 Error codes, The Server has failed to process the request.
500 Server Internal Error
The server could not fulfill the request due to some unexpected condition.
501 Not Implemented
The server does not have the ability to fulfill the request, such as because it does not recognize the request method. (Compare with 405 Method Not Allowed, where the server recognizes the method but does not allow or support it.
502 Bad Gateway
The server is acting as a gateway or proxy, and received an invalid response from a downstream server while attempting to fulfill the request.
503 Service Unavailable
The server is undergoing maintenance or is temporarily overloaded and so cannot process the request. A "Retry-After" header field may specify when the client may reattempt its request.
504 Server Time-out
The server attempted to access another server in attempting to process the request, and did not receive a prompt response.
505 Version Not Supported
The SIP protocol version in the request is not supported by the server.
513 Message Too Large
The request message length is longer than the server can process.
580 Precondition Failure
The server is unable or unwilling to meet some constraints specified in the offer.