Tools

SIP response-code lookup

Last reviewed September 4, 2026

Enter a three-digit Session Initiation Protocol (SIP) code, such as 408 or 503. The tool explains the code and gives you a useful first check.

The code tells you what one device reported. It does not always tell you which device or setting caused the problem.

SIP response-code lookup

SIP response code

Enter a three-digit SIP response code.

Code	Meaning	Check this first
100	Trying	The request arrived. Wait for the next response.
180	Ringing	The destination is ringing. Check that the caller hears the expected ringback.
183	Session Progress	Check the Session Description Protocol (SDP) and the early-media path.
200	OK	The request succeeded. For an INVITE, check the ACK and the two-way media path.
302	Moved Temporarily	Check the new address in the Contact header before retrying.
400	Bad Request	Check the request format and required headers.
401	Authentication Required	Check the username, password, realm, nonce handling, and authentication retry.
403	Forbidden	The server refused the request. Check the identity, permissions, and policy.
404	Not Found	Check the user, number, route, and domain.
407	Proxy Authentication Required	Check the proxy username, password, realm, and authentication retry.
408	Request Timeout	Check that the next device is reachable and that the route is correct.
415	Unsupported Media Type	Check the message Content-Type and whether the receiving system supports it.
420	Bad Extension	Remove or correct the unsupported SIP extension named in the response.
423	Interval Too Brief	Use a registration or session time at least as long as the Min-Expires value.
480	Temporarily Unavailable	Check registration, network access, routing, and temporary call rules.
481	Call or Transaction Not Found	Check the Call-ID, tags, CSeq, dialog details, and transaction state.
483	Too Many Hops	Look for a routing loop and check Max-Forwards at each step.
484	Address Incomplete	Check the number format, dial plan, and route.
486	Busy Here	The phone is busy or rejected the call. Check its state and call-handling rules.
487	Request Terminated	An earlier request was cancelled. Match the CANCEL with the original request.
488	Not Acceptable Here	Compare the SDP offer and answer for codecs, encryption, direction, and media support.
491	Request Pending	Another request in the same call is still running. Follow the SIP retry timing.
500	Server Internal Error	Check the responding server logs and any response from the next device.
503	Service Unavailable	Check capacity, maintenance, trunk status, routes, and the Retry-After value.
580	Precondition Failure	Check whether the required Quality of Service (QoS) or session conditions can be met.
603	Declined	The destination declined the request. Check the user choice and call policy.
604	Does Not Exist Anywhere	Check that the user or number exists in the correct domain.
606	Not Acceptable	The user cannot accept this session. Check device support and policy.

What to do next

First, find which phone, phone system, provider, or other SIP device sent the response. Then check the call direction and the logs on that device.

For a deeper investigation, compare the SIP request and response, headers, Session Description Protocol (SDP), and media path. Do not paste a real SIP trace into a public tool. A trace can contain phone numbers, network addresses, and sign-in details.