Shows that the request has been received and processing is still in progress. In voice traces this is where you see early dialog and alerting progression.
100 Trying, 180 Ringing, 183 Session ProgressIMS / SIP Responses
This page groups SIP responses the way they are usually read in IMS traces: first by response class, then by the specific code that explains what happened.
In practice, the first digit gives you the fast answer. After that, the exact code tells you whether the issue is policy, authentication, capability mismatch, temporary reachability, server trouble, or a final global stop.
Response Message Pages
Open the individual IMS / SIP response references below.
100 Trying
100 Trying confirms that the request was received and transaction processing has started.
180 Ringing180 Ringing shows that the terminating side is being alerted for the session request.
181 Call Is Being Forwarded181 Call Is Being Forwarded shows that the original target is redirecting or forwarding the session attempt.
182 Queued182 Queued shows that the request is waiting in a queue instead of progressing immediately.
183 Session Progress183 Session Progress shows that the session is advancing before final answer and often carries early-session SDP.
200 OK200 OK confirms that the SIP request completed successfully.
202 Accepted202 Accepted means the request was accepted for further handling, but the full outcome will be reported later.
302 Moved Temporarily302 Moved Temporarily redirects the sender to another temporary target or contact.
380 Alternative Service380 Alternative Service indicates that the original request cannot be served as asked and suggests another service path.
400 Bad Request400 Bad Request means the request could not be understood or accepted because of syntax or structural problems.
401 Unauthorized401 Unauthorized challenges the request and asks the sender to retry with authentication.
403 Forbidden403 Forbidden means the request was understood but the network or service refused to allow it.
404 Not Found404 Not Found means the target user or service could not be found at the requested destination.
405 Method Not Allowed405 Method Not Allowed means the target exists but does not allow the SIP method that was used.
406 Not Acceptable406 Not Acceptable means the request was understood, but the target cannot satisfy the acceptable formats or features indicated.
407 Proxy Authentication Required407 Proxy Authentication Required challenges the sender to authenticate through a SIP proxy, commonly the P-CSCF path.
408 Request Timeout408 Request Timeout means the expected transaction progress did not happen in time.
415 Unsupported Media Type415 Unsupported Media Type means the target cannot process the request body in the format that was sent.
480 Temporarily Unavailable480 Temporarily Unavailable means the target is currently unreachable or unavailable, but not permanently unknown.
481 Call or Transaction Does Not Exist481 Call or Transaction Does Not Exist means the receiving side could not match the request to a valid dialog or transaction.
486 Busy Here486 Busy Here means the target user is busy at the reached destination.
487 Request Terminated487 Request Terminated means the original request branch was stopped before normal completion.
488 Not Acceptable Here488 Not Acceptable Here means the request reached the target, but the local media or session offer cannot be accepted there.
500 Server Internal Error500 Server Internal Error means the server hit an internal problem while handling the request.
503 Service Unavailable503 Service Unavailable means the service cannot handle the request right now, usually for temporary server or overload reasons.
504 Server Time-out504 Server Time-out means a server did not get a timely answer from another server while handling the request.
600 Busy Everywhere600 Busy Everywhere means the request should not be retried elsewhere because the target is considered busy globally.
603 Decline603 Decline means the request was actively refused and should not be retried to other locations for the same attempt.
604 Does Not Exist Anywhere604 Does Not Exist Anywhere means the target does not exist globally for the request.
606 Not Acceptable606 Not Acceptable means no acceptable target branch can satisfy the requested session or content constraints.
608 Rejected608 Rejected means the request was rejected without exposing the more specific target-state reason.
Response Classes
Confirms that the request completed successfully. In IMS, 200 OK is the response most people check first.
200 OK, 202 AcceptedTells the sender to try a different target or route. These are less common in everyday IMS traces but still useful in service-routing cases.
302 Moved Temporarily, 380 Alternative ServiceThe request reached the far side, but that side rejected it because of authorization, syntax, policy, capability, routing, or request-state problems.
401 Unauthorized, 403 Forbidden, 404 Not Found, 408 Request Timeout, 480 Temporarily Unavailable, 488 Not Acceptable HereThe receiving side could not process the request because of an internal or temporary network-side problem.
500 Server Internal Error, 503 Service Unavailable, 504 Server Time-outThe request should not be retried against other locations because the failure is considered global for that request.
600 Busy Everywhere, 603 Decline, 604 Does Not Exist Anywhere