Direct answer
SIP response codes are three-digit status messages used during VoIP call setup and teardown. 1xx means the call is still processing, 2xx means success, 3xx means redirection, 4xx usually points to caller-side or configuration issues, 5xx points to server-side failures, and 6xx means a global call failure.
Common SIP response code answers
- What does SIP 100 Trying mean? SIP 100 Trying means the server received the request and is working on it. It is an informational response, not a final answer.
- What does SIP 180 Ringing mean? SIP 180 Ringing means the destination endpoint is being alerted and the callee's phone is ringing.
- What does SIP 183 Session Progress mean? SIP 183 Session Progress means call setup is progressing and early media, such as ringback or announcements, may be available.
- What does SIP 480 Temporarily Unavailable mean? SIP 480 Temporarily Unavailable means the destination exists but cannot take the call right now.
- What does SIP 486 Busy Here mean? SIP 486 Busy Here means the destination was reached but the user or endpoint is currently busy.
- What does SIP 487 Request Terminated mean? SIP 487 Request Terminated means the request was canceled before it completed, often because the caller hung up or a CANCEL was sent.
- What does SIP 488 Not Acceptable Here mean? SIP 488 Not Acceptable Here usually means the endpoint rejected the proposed media, codec, or session parameters.
- What does SIP 503 Service Unavailable mean? SIP 503 Service Unavailable means the SIP service, proxy, gateway, or upstream provider is down, overloaded, or temporarily unable to handle the request.
- What does SIP 603 Decline mean? SIP 603 Decline means the callee or destination rejected the call globally, so another branch is unlikely to answer.
1xx SIP Response Codes
1xx = Informational. The call request is still being processed. Nothing has finally succeeded or failed yet.
Use 1xx responses to understand call setup progress before the final answer, failure, or redirect arrives.
| Code | Meaning | Plain English |
|---|---|---|
100 |
Trying | Working on it. |
180 |
Ringing | The person's phone is ringing. |
181 |
Call Is Being Forwarded | The call is being forwarded elsewhere. |
182 |
Queued | The call is waiting in line. |
183 |
Session Progress | Call setup is progressing. |
199 |
Early Dialog Terminated | The setup process stopped. |
2xx SIP Response Codes
2xx = Success. The request was accepted or completed successfully.
A 2xx response usually means the SIP action worked. For calls, 200 OK is the final success response.
| Code | Meaning | Plain English |
|---|---|---|
200 |
OK | Everything worked. |
202 |
Accepted | We got it and will process it. |
204 |
No Notification | Request succeeded silently. |
3xx SIP Response Codes
3xx = Redirection. The request should be sent somewhere else.
Check the Contact header, routing policy, proxy behavior, and whether the redirected destination is trusted.
| Code | Meaning | Plain English |
|---|---|---|
300 |
Multiple Choices | More than one place can answer. |
301 |
Moved Permanently | This number/user moved permanently. |
302 |
Moved Temporarily | Temporarily redirected. |
305 |
Use Proxy | You must route through another server. |
380 |
Alternative Service | Use another service instead. |
4xx SIP Response Codes
4xx = Client Errors. The request failed because of caller-side input, authentication, routing, addressing, permission, or media negotiation.
Start with the dialed address, authentication credentials, registration state, caller ID rules, codecs, and expiry timers.
| Code | Meaning | Plain English |
|---|---|---|
400 |
Bad Request | The request was broken. |
401 |
Unauthorized | Login required. |
402 |
Payment Required | Payment needed. |
403 |
Forbidden | You're not allowed. |
404 |
Not Found | Destination doesn't exist. |
405 |
Method Not Allowed | That action isn't allowed. |
406 |
Not Acceptable | The system can't handle this format. |
407 |
Proxy Authentication Required | Authenticate with the proxy. |
408 |
Request Timeout | The other side didn't answer in time. |
409 |
Conflict | This request conflicts with current state. |
410 |
Gone | This destination no longer exists. |
411 |
Length Required | A required size field is missing. |
413 |
Request Entity Too Large | The request is too big. |
414 |
Request URI Too Long | The SIP address is too long. |
415 |
Unsupported Media Type | Codec or media type not supported. |
416 |
Unsupported URI Scheme | Unsupported address format. |
417 |
Unknown Resource Priority | Unknown priority setting. |
420 |
Bad Extension | Feature not supported. |
421 |
Extension Required | A required SIP feature is missing. |
422 |
Session Interval Too Small | Session refresh interval is too short. |
423 |
Interval Too Brief | The timeout value is too small. |
424 |
Bad Location Information | Location info is incorrect. |
428 |
Use Identity Header | Identity verification required. |
429 |
Provide Referrer Identity | Need referral identity. |
430 |
Flow Failed | The network connection failed. |
433 |
Anonymity Disallowed | Anonymous calling not allowed. |
436 |
Bad Identity Info | Caller identity information failed. |
437 |
Unsupported Certificate | Certificate isn't supported. |
438 |
Invalid Identity Header | Identity check failed. |
439 |
First Hop Lacks Outbound Support | First server doesn't support outbound. |
440 |
Max-Breadth Exceeded | Too many routing paths. |
469 |
Bad Info Package | Unsupported INFO message type. |
470 |
Consent Needed | User permission required. |
480 |
Temporarily Unavailable | The person can't take the call right now. |
481 |
Call/Transaction Does Not Exist | That call session doesn't exist. |
482 |
Loop Detected | The call is stuck looping. |
483 |
Too Many Hops | The call bounced too many times. |
484 |
Address Incomplete | The phone number is incomplete. |
485 |
Ambiguous | The destination is unclear. |
486 |
Busy Here | The person is busy. |
487 |
Request Terminated | The call was canceled. |
488 |
Not Acceptable Here | Media or codec not acceptable. |
489 |
Bad Event | Unsupported subscription/event. |
491 |
Request Pending | Wait until current request finishes. |
493 |
Undecipherable | The message couldn't be decrypted. |
494 |
Security Agreement Required | A security agreement is needed. |
5xx SIP Response Codes
5xx = Server Errors. The request reached a server, but the server or upstream path failed.
Check provider status, SBC or proxy health, upstream timeout logs, failover routing, and overload controls.
| Code | Meaning | Plain English |
|---|---|---|
500 |
Server Internal Error | Something broke on the server. |
501 |
Not Implemented | The server doesn't support this. |
502 |
Bad Gateway | Another server failed. |
503 |
Service Unavailable | The service is unavailable. |
504 |
Server Time-out | Another server took too long. |
505 |
Version Not Supported | Wrong SIP version. |
513 |
Message Too Large | The SIP message is too big. |
555 |
Push Notification Service Not Supported | Push notifications unsupported. |
580 |
Precondition Failure | Required setup conditions failed. |
6xx SIP Response Codes
6xx = Global Failures. The call failed globally, so retrying another branch usually will not help.
Treat 6xx responses as decisive call failures. Review user rejection, spam policy, identity rules, or global media compatibility.
| Code | Meaning | Plain English |
|---|---|---|
600 |
Busy Everywhere | The person is busy on all devices. |
603 |
Decline | The person rejected the call. |
604 |
Does Not Exist Anywhere | That number/user doesn't exist anywhere. |
606 |
Not Acceptable | No compatible media found. |
607 |
Unwanted | The call was marked as spam/unwanted. |
608 |
Rejected | The call was blocked everywhere. |
SIP response code FAQ
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What are SIP response codes?
SIP response codes are three-digit status messages used by SIP servers, proxies, and endpoints during VoIP call setup, routing, authentication, media negotiation, and call teardown.
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How do you read SIP response code classes?
Read the first digit first: 1xx is informational, 2xx is success, 3xx is redirection, 4xx is usually a client-side or configuration problem, 5xx is a server-side failure, and 6xx is a global failure.
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Is SIP 486 Busy Here a network failure?
No. SIP 486 Busy Here normally means the destination endpoint was reached, but the user is busy. It is different from server failures like 503 or routing failures like 404.
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What is the difference between SIP 480 and SIP 486?
SIP 480 Temporarily Unavailable means the destination cannot take the call right now. SIP 486 Busy Here is more specific: the destination is available enough to respond, but the user or device is busy.
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What SIP code usually means a codec mismatch?
SIP 488 Not Acceptable Here commonly points to media negotiation failure, such as no compatible codec, unsupported SDP, or unacceptable media settings.
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Should you retry a call after SIP 503 Service Unavailable?
Usually yes, but with backoff or failover routing. SIP 503 often means a temporary server, gateway, or provider-side availability issue rather than a permanent destination problem.
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What is the difference between SIP 4xx, 5xx, and 6xx errors?
4xx errors usually point to the request, caller, destination, authentication, or media setup. 5xx errors point to a server or upstream provider. 6xx errors are global failures that usually stop further routing attempts.