SIP error codes are the 4xx, 5xx and 6xx responses a SIP server returns when a call cannot be completed: 4xx means the request failed at the far end or was wrong, 5xx means a server or gateway failed, and 6xx means a global failure. The table below lists the response codes defined in RFC 3261 with their meaning and typical cause on a VoIP route.
What are the SIP response code classes?
Every SIP response has a three-digit code. The first digit gives the class: 1xx provisional (the call is progressing), 2xx success, 3xx redirection, 4xx client failure, 5xx server failure, 6xx global failure. Only 3xx to 6xx end a call attempt, so these are the codes you see in CDRs for failed calls.
SIP response codes list
| Code | Name | Meaning | Typical cause |
|---|---|---|---|
| 100 | Trying | Request received, being processed | Normal, sent by the next hop |
| 180 | Ringing | Destination is alerting | Normal |
| 181 | Call Is Being Forwarded | Call forwarded elsewhere | Forwarding at the far end |
| 182 | Queued | Callee unavailable, call queued | Queue at the far end |
| 183 | Session Progress | Progress information, often early media | Ringback tone or announcement |
| 200 | OK | Request succeeded | Call answered |
| 300 | Multiple Choices | Several possible locations | Redirect server |
| 301 | Moved Permanently | User moved to a new address | Number or URI changed |
| 302 | Moved Temporarily | Retry at the given contact | Redirect-based routing |
| 305 | Use Proxy | Must go through a proxy | Rare in practice |
| 380 | Alternative Service | Call failed, alternatives possible | Rare in practice |
| 400 | Bad Request | Malformed request | Syntax or header error |
| 401 | Unauthorized | User authentication required | Missing or wrong credentials |
| 402 | Payment Required | Reserved for future use | Some switches use it for no credit |
| 403 | Forbidden | Understood but refused | IP not allowed, destination blocked, no balance |
| 404 | Not Found | User does not exist at the domain | Wrong number, prefix or format |
| 405 | Method Not Allowed | Method not supported for this address | Configuration mismatch |
| 406 | Not Acceptable | Content not acceptable | Accept header mismatch |
| 407 | Proxy Authentication Required | Proxy wants credentials | Normal challenge, or wrong password |
| 408 | Request Timeout | No response in time | Unreachable or slow next hop |
| 410 | Gone | User no longer available | Disconnected number |
| 413 | Request Entity Too Large | Body too large | Oversized SDP |
| 414 | Request-URI Too Long | URI too long | Bad dialplan output |
| 415 | Unsupported Media Type | Body format not supported | Wrong content type |
| 416 | Unsupported URI Scheme | URI scheme not understood | tel or sips not supported |
| 420 | Bad Extension | Required extension unknown | Require header mismatch |
| 421 | Extension Required | Server needs an extension | Missing Supported header |
| 423 | Interval Too Brief | Registration expiry too short | Low Expires value |
| 480 | Temporarily Unavailable | Callee reached but unavailable | Phone off, no answer, no route |
| 481 | Call/Transaction Does Not Exist | Unknown dialog or transaction | Lost state, late BYE |
| 482 | Loop Detected | Request looped | Routing loop between carriers |
| 483 | Too Many Hops | Max-Forwards reached zero | Routing loop or long chain |
| 484 | Address Incomplete | Number too short | Missing digits |
| 485 | Ambiguous | Address is ambiguous | Rare in practice |
| 486 | Busy Here | Callee busy at this endpoint | Line busy |
| 487 | Request Terminated | Request cancelled | Caller hung up before answer |
| 488 | Not Acceptable Here | Session description not acceptable | No common codec |
| 491 | Request Pending | Another request in progress | Simultaneous re-INVITEs |
| 493 | Undecipherable | Encrypted body cannot be read | S/MIME key problem |
| 500 | Server Internal Error | Unexpected server condition | Switch fault or unmapped cause |
| 501 | Not Implemented | Method not supported | Feature not supported |
| 502 | Bad Gateway | Invalid response from downstream | Next carrier failed |
| 503 | Service Unavailable | Overloaded or in maintenance | No capacity, CPS or channel limit |
| 504 | Server Time-out | No timely response from downstream | Next carrier did not answer |
| 505 | Version Not Supported | SIP version not supported | Very old equipment |
| 513 | Message Too Large | Message exceeds server limit | Oversized packet over UDP |
| 600 | Busy Everywhere | Callee busy on all endpoints | User busy |
| 603 | Decline | Callee does not want to take the call | Call rejected by user |
| 604 | Does Not Exist Anywhere | User does not exist anywhere | Invalid number |
| 606 | Not Acceptable | Session not acceptable | Media not supported |
Later RFCs added more codes (for example 422 Session Interval Too Small, from RFC 4028). Carriers also map telephone network causes to SIP codes, so the Q.850 cause code in the Reason header often tells you more than the SIP code alone.
What does SIP 503 mean?
503 Service Unavailable means the server is temporarily unable to handle the call, because of overload or maintenance. On wholesale routes it usually means no capacity: the channel or CPS limit is reached, no supplier is available for the destination, or the account is blocked. Details and fixes: SIP 503 Service Unavailable.
What does SIP 487 mean?
487 Request Terminated is the response to an INVITE that was cancelled before answer, usually because the caller hung up or a dialer stopped ringing. It is normal, but a high share points to short ring timeouts or high post-dial delay. See SIP 487 Request Terminated.
What do SIP 408, 480 and 486 mean?
408 Request Timeout means no final response arrived in time. 480 Temporarily Unavailable means the callee was reached but cannot take the call now. 486 Busy Here means the line is busy. All three affect ASR. See SIP 408, 480 and 486 explained, and read ASR, ACD and PDD to see how these codes shape route statistics.
Reading SIP codes on your routes
Group failed calls by SIP code per destination and per supplier in your CDRs. A sudden jump in 503 or 403 is usually a supplier or account issue; a jump in 404 or 484 is usually a number format issue on your side. If a route keeps failing, VoIPMatcher can source an alternative VoIP termination route from its carriers: request a VoIP route.
Frequently asked questions
What is SIP 403 Forbidden?
403 Forbidden means the server understood the request but refuses to process it, and retrying with the same credentials will not help. On VoIP routes the usual causes are a source IP that is not whitelisted, a destination the account is not allowed to call, an exhausted prepaid balance, or a caller ID the carrier rejects.
What is SIP 404 Not Found?
404 Not Found means the server is sure the called user does not exist at that domain. On outbound calls it almost always points to a number problem: a wrong country code, a missing or extra technical prefix, a non E.164 format, or a number that is not assigned on the destination network.
What is the difference between SIP 401 and 407?
Both ask for authentication. 401 Unauthorized comes from the destination server, usually a registrar, while 407 Proxy Authentication Required comes from a proxy on the path. A single 401 or 407 followed by a new INVITE with credentials is normal. Repeated challenges mean the username, password or realm is wrong.
What is SIP 488 Not Acceptable Here?
488 means the far end cannot accept the session described in the SDP, most often because there is no codec in common. Check that your offer includes a codec the route supports, typically G.711 or G.729, and that DTMF and fax settings (RFC 2833 or T.38) match what the supplier expects.
Which SIP codes count as failed calls in ASR?
ASR counts answered calls (200 OK) against all attempts, so every final 3xx, 4xx, 5xx or 6xx response lowers it. Many carriers also report NER, which treats user-side results such as busy, no answer and call rejected as successful network outcomes, so it isolates failures caused by the network and the route itself.
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