# SIP response codes (1xx-6xx)

> SIP responses are three-digit codes in six classes, mirroring HTTP: 1xx provisional (request received, still processing), 2xx success, 3xx redirection, 4xx client/request failure, 5xx server failure, and 6xx global failure (the request fails everywhere). The first digit sets the class; the full code identifies the specific outcome.

- Canonical URL: https://www.broadnet.me/reference/sip-response-codes
- Language: en
- Last updated: 2026-07-18
- Publisher: BroadNet (https://www.broadnet.me)

The six classes of SIP response code and the ones that matter: 1xx provisional (100 Trying, 180 Ringing), 2xx success (200 OK), 3xx redirection, 4xx client error (404, 486), 5xx server error and 6xx global failure.

## The six response classes

Every SIP response carries a status code whose leading digit defines its class, exactly as in HTTP. Provisional (1xx) responses are interim — the request is progressing but no final outcome yet. 2xx is success. 3xx redirects the caller elsewhere. 4xx means the request failed and is usually the caller's or request's fault. 5xx means a server failed to fulfil an apparently valid request. 6xx is a definitive global failure — the request will fail at every location, so proxies should stop trying alternatives.

*SIP response classes*

| Class | Meaning |
|---|---|
| 1xx | Provisional — received, still processing (not final) |
| 2xx | Success — the request succeeded |
| 3xx | Redirection — try another location |
| 4xx | Client/request failure — cannot be fulfilled as sent |
| 5xx | Server failure — server could not fulfil a valid request |
| 6xx | Global failure — fails everywhere; stop forking |

## Provisional and success codes

The early call flow is driven by 1xx and 2xx. 100 Trying tells the upstream hop the request was received and stops retransmissions. 180 Ringing indicates the callee's phone is alerting. 183 Session Progress carries early media (ringback or announcements) before the call is answered. 200 OK is the definitive success — for an INVITE it means the call was answered and it carries the SDP answer, and it must be confirmed with ACK.

*Key 1xx and 2xx codes*

| Code | Reason | Meaning |
|---|---|---|
| 100 | Trying | Request received; stops retransmission upstream |
| 180 | Ringing | Callee is being alerted |
| 181 | Call Is Being Forwarded | Request is being redirected |
| 182 | Queued | Callee temporarily unavailable; call queued |
| 183 | Session Progress | Early media (ringback/announcement) available |
| 200 | OK | Success — for INVITE, the call is answered |

## Client failures (4xx) — the ones you will debug

4xx codes dominate real-world troubleshooting. 401/407 request authentication (from a UAS/registrar or a proxy respectively). 403 Forbidden and 404 Not Found are self-explanatory. 408 Request Timeout means no response in time. 486 Busy Here is the callee busy; 480 Temporarily Unavailable is the callee unreachable. 487 Request Terminated is the response to an INVITE that was CANCELled. 488 Not Acceptable Here usually signals SDP/codec negotiation failure.

*Frequently seen 4xx codes*

| Code | Reason | Typical cause |
|---|---|---|
| 400 | Bad Request | Malformed message |
| 401 / 407 | Unauthorized / Proxy Auth Required | Authentication challenge (UAS / proxy) |
| 403 | Forbidden | Authenticated but not permitted |
| 404 | Not Found | Unknown user / bad Request-URI |
| 408 | Request Timeout | No response within the timer |
| 480 | Temporarily Unavailable | Registered but not reachable now |
| 486 | Busy Here | Callee is busy |
| 487 | Request Terminated | INVITE was CANCELled |
| 488 | Not Acceptable Here | Media/codec (SDP) negotiation failed |

## Server (5xx) and global (6xx) failures

5xx and 6xx signal failures beyond the request itself. 500 Server Internal Error and 503 Service Unavailable indicate a server or downstream problem; 504 Server Time-out is a gateway timeout. The 6xx class is emphatic: 600 Busy Everywhere, 603 Decline (the callee actively rejected), 604 Does Not Exist Anywhere, and 606 Not Acceptable. Because 6xx means the request fails at every possible location, a forking proxy stops trying other branches — unlike a 4xx, which is specific to one target.

*Key 5xx and 6xx codes*

| Code | Reason | Meaning |
|---|---|---|
| 500 | Server Internal Error | Server hit an unexpected error |
| 503 | Service Unavailable | Server temporarily overloaded/down |
| 504 | Server Time-out | No timely response from a downstream server |
| 600 | Busy Everywhere | Callee busy at all locations |
| 603 | Decline | Callee explicitly declined the call |
| 604 | Does Not Exist Anywhere | User exists nowhere |
| 606 | Not Acceptable | Session parameters unacceptable everywhere |

## Frequently asked questions

### What does SIP 200 OK mean?

200 OK is the success response. For an INVITE it means the call was answered and it carries the SDP answer that completes media negotiation; the caller must confirm it with an ACK. For other requests it simply confirms the operation succeeded.

### What is the difference between 486 Busy Here and 600 Busy Everywhere?

486 Busy Here means the specific contacted endpoint is busy, so a forking proxy may still try other locations. 600 Busy Everywhere is a global (6xx) failure meaning the callee is busy at all locations, so proxies should stop forking and return the failure.

### Why am I getting SIP 488 Not Acceptable Here?

488 almost always means media negotiation failed — the SDP offer and answer share no common codec, or a required media parameter (for example an SRTP crypto suite) could not be agreed. Check that both sides support a common codec and transport.

### What does 407 Proxy Authentication Required mean?

A proxy is challenging the request for credentials. The client should resend the request with a Proxy-Authorization header computed from the challenge. It is the proxy-side counterpart of 401 Unauthorized, which comes from the UAS or registrar.

### What is SIP response 100 Trying for?

100 Trying is a provisional response telling the upstream hop that the request was received and is being processed, which stops the sender retransmitting the request. It is hop-by-hop and never travels end-to-end like 180 or 200.

## Sources

- [IETF RFC 3261 — SIP response codes (§21)](https://datatracker.ietf.org/doc/html/rfc3261#section-21)
- [IANA — SIP Response Codes registry](https://www.iana.org/assignments/sip-parameters/sip-parameters.xhtml)

## Related

- [SIP methods and requests](https://www.broadnet.me/reference/sip-methods-and-requests)
- [SIP call flow](https://www.broadnet.me/reference/sip-call-flow)
- [SIP headers](https://www.broadnet.me/reference/sip-headers)
- [SIP protocol overview](https://www.broadnet.me/reference/sip-protocol-overview)
- [Programmable voice (product)](https://www.broadnet.me/products/voice)