Simple definition
SIP response codes classify a request outcome: 1xx provisional, 2xx success, 3xx redirection, and 4xx to 6xx failures.
Purpose and use
A SIP response code describes progress or the result of a signaling request.
How does it work?
The 1xx family is provisional, 2xx means success, 3xx redirection, 4xx request failure, 5xx server failure and 6xx global failure.
Example and verification
For a rejected call, identify the exact code, the request it answers and the device that generated or translated it.
Points to watch
A PBX's displayed label can simplify the real cause. A successful final response does not guarantee correct media forwarding.
In short
Interpret codes within the signaling sequence and the relevant component's logs, rather than only as an on-screen label.