SIP response codes: what each one means
A SIP response code is the three-digit number a server sends back to a call: 1xx while it is in progress, 2xx when it connects, and 3xx to 6xx when it does not. This is the full list from the IANA registry, with how Monitor reads each one on a test call.
How do you read a SIP code?
The first digit gives the class: 4xx is a refusal of the request (number does not exist, busy, credentials), 5xx is a failure of the server that tried to connect the call (overload, route, maintenance) and 6xx is a final refusal at any destination. The Q.850 cause that usually comes with it, in the Reason header, gives the reason on the telephone network's side.
In the "Reading on a test call" column, Down means the call counts as a circuit failure; Monitor repeats it every 2 minutes before it opens an incident. "Depends" means the reading changes with what should answer the number, and the code's page explains each case. 1xx and 2xx codes are not failures.
1xx: provisional (the call is in progress)
| Code | Official name | Defined in | Reading on a test call |
|---|---|---|---|
| SIP 100 | Trying | RFC 3261 | Not a failure |
| SIP 180 | Ringing | RFC 3261 | Not a failure |
| SIP 181 | Call Is Being Forwarded | RFC 3261 | Not a failure |
| SIP 182 | Queued | RFC 3261 | Not a failure |
| SIP 183 | Session Progress | RFC 3261 | Not a failure |
| SIP 199 | Early Dialog Terminated | RFC 6228 | Not a failure |
2xx: success
| Code | Official name | Defined in | Reading on a test call |
|---|---|---|---|
| SIP 200 | OK | RFC 3261 | Not a failure |
| SIP 202 | Accepted (Deprecated) | RFC 6665 | Not a failure |
| SIP 204 | No Notification | RFC 5839 | Not a failure |
3xx: redirection
| Code | Official name | Defined in | Reading on a test call |
|---|---|---|---|
| SIP 300 | Multiple Choices | RFC 3261 | Down |
| SIP 301 | Moved Permanently | RFC 3261 | Down |
| SIP 302 | Moved Temporarily | RFC 3261 | Down |
| SIP 305 | Use Proxy | RFC 3261 | Down |
| SIP 380 | Alternative Service | RFC 3261 | Down |
4xx: request failure
| Code | Official name | Defined in | Reading on a test call |
|---|---|---|---|
| SIP 400 | Bad Request | RFC 3261 | Down |
| SIP 401 | Unauthorized | RFC 3261 | Monitoring blind |
| SIP 402 | Payment Required | RFC 3261 | Down |
| SIP 403 | Forbidden | RFC 3261 | Down |
| SIP 404 | Not Found | RFC 3261 | Down |
| SIP 405 | Method Not Allowed | RFC 3261 | Down |
| SIP 406 | Not Acceptable | RFC 3261 | Down |
| SIP 407 | Proxy Authentication Required | RFC 3261 | Monitoring blind |
| SIP 408 | Request Timeout | RFC 3261 | Down |
| SIP 410 | Gone | RFC 3261 | Down |
| SIP 412 | Conditional Request Failed | RFC 3903 | Down |
| SIP 413 | Request Entity Too Large | RFC 3261 | Down |
| SIP 414 | Request-URI Too Long | RFC 3261 | Down |
| SIP 415 | Unsupported Media Type | RFC 3261 | Down |
| SIP 416 | Unsupported URI Scheme | RFC 3261 | Down |
| SIP 417 | Unknown Resource-Priority | RFC 4412 | Down |
| SIP 420 | Bad Extension | RFC 3261 | Down |
| SIP 421 | Extension Required | RFC 3261 | Down |
| SIP 422 | Session Interval Too Small | RFC 4028 | Down |
| SIP 423 | Interval Too Brief | RFC 3261 | Down |
| SIP 424 | Bad Location Information | RFC 6442 | Down |
| SIP 425 | Bad Alert Message | RFC 8876 | Down |
| SIP 428 | Use Identity Header | RFC 8224 | Down |
| SIP 429 | Provide Referrer Identity | RFC 3892 | Down |
| SIP 430 | Flow Failed | RFC 5626 | Down |
| SIP 433 | Anonymity Disallowed | RFC 5079 | Down |
| SIP 436 | Bad Identity Info | RFC 8224 | Down |
| SIP 437 | Unsupported Credential | RFC 8224 | Down |
| SIP 438 | Invalid Identity Header | RFC 8224 | Down |
| SIP 439 | First Hop Lacks Outbound Support | RFC 5626 | Down |
| SIP 440 | Max-Breadth Exceeded | RFC 5393 | Down |
| SIP 469 | Bad Info Package | RFC 6086 | Down |
| SIP 470 | Consent Needed | RFC 5360 | Down |
| SIP 480 | Temporarily Unavailable | RFC 3261 | Depends on the circuit |
| SIP 481 | Call/Transaction Does Not Exist | RFC 3261 | Down |
| SIP 482 | Loop Detected | RFC 3261 | Down |
| SIP 483 | Too Many Hops | RFC 3261 | Down |
| SIP 484 | Address Incomplete | RFC 3261 | Down |
| SIP 485 | Ambiguous | RFC 3261 | Down |
| SIP 486 | Busy Here | RFC 3261 | Depends on the circuit |
| SIP 487 | Request Terminated | RFC 3261 | Depends on the circuit |
| SIP 488 | Not Acceptable Here | RFC 3261 | Down |
| SIP 489 | Bad Event | RFC 6665 | Down |
| SIP 491 | Request Pending | RFC 3261 | Down |
| SIP 493 | Undecipherable | RFC 3261 | Down |
| SIP 494 | Security Agreement Required | RFC 3329 | Down |
5xx: server failure
| Code | Official name | Defined in | Reading on a test call |
|---|---|---|---|
| SIP 500 | Server Internal Error | RFC 3261 | Down |
| SIP 501 | Not Implemented | RFC 3261 | Down |
| SIP 502 | Bad Gateway | RFC 3261 | Down |
| SIP 503 | Service Unavailable | RFC 3261 | Down |
| SIP 504 | Server Time-out | RFC 3261 | Down |
| SIP 505 | Version Not Supported | RFC 3261 | Down |
| SIP 513 | Message Too Large | RFC 3261 | Down |
| SIP 555 | Push Notification Service Not Supported | RFC 8599 | Down |
| SIP 580 | Precondition Failure | RFC 3312 | Down |
6xx: global failure
| Code | Official name | Defined in | Reading on a test call |
|---|---|---|---|
| SIP 600 | Busy Everywhere | RFC 3261 | Depends on the circuit |
| SIP 603 | Decline | RFC 3261 | Depends on the circuit |
| SIP 604 | Does Not Exist Anywhere | RFC 3261 | Down |
| SIP 606 | Not Acceptable | RFC 3261 | Down |
| SIP 607 | Unwanted | RFC 8197 | Down |
| SIP 608 | Rejected | RFC 8688 | Down |
Sources: IANA SIP response code registry and RFC 3261, checked on October 7, 2026. See also the list of Q.850 causes and the SIP code lookup.
Know which code took your trunk down, as it happens
Monitor calls every voice circuit all day and delivers each failure with the SIP code and the Q.850 cause as evidence.