SIP response codes
Every SIP request gets a three-digit reply. The first digit says what kind of answer it is: 1xx still working, 2xx done, 3xx look elsewhere, 4xx a problem with the request or the called side, 5xx a server failed, 6xx refused everywhere. Type the code from your log to see what it means.
The session description cannot be accepted, most often because no offered codec is supported.
Causes and fixes for 48860 codes listed, 14 with full causes and fixes. Part of our SIP troubleshooting guide. Updated 10 November 2026.
The six classes
Read the first digit before anything else.
A code a phone does not know is treated as the first code of its class, so a 499 reads as a 400. The class alone tells you whose side to look at.
- 1xx
Provisional
The request was received and is being processed. Not an error.
Nothing to fix - 2xx
Success
The request worked.
Nothing to fix - 3xx
Redirection
Try somewhere else. Trunks rarely send these to customers.
Rarely seen on a trunk - 4xx
Client error
Something about the request is wrong, or the called party cannot take it.
Your request, or the called side - 5xx
Server error
A server along the path failed or is overloaded.
A server on the path - 6xx
Global failure
The called party refused, and trying elsewhere will not help.
The called side, for good
The full list
Every code you are likely to meet, grouped by class.
Codes with a coral key have a full entry. Before changing settings, find the code in the SIP log: reading the SIP log.
1xx Provisional
- 100TryingRFC 3261
The next server has the request and is working on it. It stops your phone from resending the INVITE.
- 180RingingRFC 3261
The called phone is ringing. Your phone usually plays its own ringback tone.
- 181Call Is Being ForwardedRFC 3261
The call is being forwarded to another destination.
- 182QueuedRFC 3261
The called party is busy but the call has been queued instead of rejected.
- 183Session ProgressRFC 3261
Early media: the network is sending audio before the call is answered, such as ringback or a recorded announcement.
- 199Early Dialog TerminatedRFC 6228
One forked branch of the call ended before answer while others may continue.
2xx Success
- 200OKRFC 3261
Success. For an INVITE it means the call was answered, and billing normally starts here.
- 202AcceptedRFC 3265
The request was accepted for processing. Seen with REFER and subscriptions.
- 204No NotificationRFC 5839
A subscription was accepted but no notification will be sent.
3xx Redirection
- 300Multiple ChoicesRFC 3261
The address resolves to several possible destinations.
- 301Moved PermanentlyRFC 3261
The user is no longer at this address; use the one in the Contact header.
- 302Moved TemporarilyRFC 3261
Retry at the address in the Contact header. Used for call forwarding by some servers.
- 305Use ProxyRFC 3261
The request must go through the proxy named in the Contact header.
- 380Alternative ServiceRFC 3261
The call failed, but an alternative service is possible.
4xx Client error
- 400Bad RequestRFC 3261
The request could not be understood, usually because of malformed syntax or a missing header.
- 401UnauthorizedRFC 3261
The server wants your credentials. Normal on the first attempt; an error only if it repeats after your phone answers the challenge.
- 402Payment RequiredRFC 3261
Reserved for future use in RFC 3261. Some providers use it for insufficient credit.
- 403ForbiddenRFC 3261
The server understood the request and refuses it. Sending credentials again will not help.
- 404Not FoundRFC 3261
The called number or user does not exist at this server.
- 405Method Not AllowedRFC 3261
The method is known but not allowed for this address. The Allow header lists what is.
- 406Not AcceptableRFC 3261
The response could only be generated in a form the request did not accept.
- 407Proxy Authentication RequiredRFC 3261
Like 401, but the challenge comes from a proxy. Normal once; an error if it repeats.
- 408Request TimeoutRFC 3261
No final response arrived in time. Often a network or reachability problem rather than a refusal.
- 410GoneRFC 3261
The user existed but is no longer available here, and no forwarding address is known.
- 413Request Entity Too LargeRFC 3261
The request body is larger than the server will accept.
- 414Request-URI Too LongRFC 3261
The address in the request line is longer than the server will process.
- 415Unsupported Media TypeRFC 3261
The body format is not supported, for example an unexpected SDP or content type.
- 416Unsupported URI SchemeRFC 3261
The URI scheme in the request (such as tel: or sips:) is not supported here.
- 420Bad ExtensionRFC 3261
The server does not support an extension listed in the Proxy-Require or Require header.
- 421Extension RequiredRFC 3261
The server needs an extension the request did not declare.
- 422Session Interval Too SmallRFC 4028
The session timer interval is shorter than the server allows. Retry with the value in Min-SE.
- 423Interval Too BriefRFC 3261
The registration expiry is too short. Retry with at least the value in Min-Expires.
- 429Provide Referrer IdentityRFC 3892
A REFER needs a Referred-By token.
- 480Temporarily UnavailableRFC 3261
The called party was reached but is not available right now, for example a phone that is off or out of coverage.
- 481Call/Transaction Does Not ExistRFC 3261
A request referred to a call or transaction the server does not know, often a BYE for a call that already ended.
- 482Loop DetectedRFC 3261
The request came back to a server it already passed through.
- 483Too Many HopsRFC 3261
The request passed through too many servers (Max-Forwards reached zero), usually a routing loop.
- 484Address IncompleteRFC 3261
The number is too short to route. Common when digits are missing or overlap dialling is used.
- 485AmbiguousRFC 3261
The address matches several possible users. The response may list them.
- 486Busy HereRFC 3261
The called party is busy at this location.
- 487Request TerminatedRFC 3261
The call was cancelled before answer, usually because the caller hung up while it was ringing.
- 488Not Acceptable HereRFC 3261
The session description cannot be accepted, most often because no offered codec is supported.
- 489Bad EventRFC 6665
The server does not understand the event package in a SUBSCRIBE or NOTIFY.
- 491Request PendingRFC 3261
Two re-INVITEs crossed. Each side retries after a random delay.
- 493UndecipherableRFC 3261
The request had an encrypted body the server could not decrypt.
- 494Security Agreement RequiredRFC 3329
The server requires a security mechanism to be negotiated first.
5xx Server error
- 500Server Internal ErrorRFC 3261
A server hit an unexpected condition. The failure is on the server side, not in your request.
- 501Not ImplementedRFC 3261
The server does not support the method used.
- 502Bad GatewayRFC 3261
A gateway received an invalid response from the next server.
- 503Service UnavailableRFC 3261
The server is temporarily overloaded or down for maintenance. A Retry-After header may say when to try again.
- 504Server Time-outRFC 3261
A server did not get a timely response from a server further along the path.
- 505Version Not SupportedRFC 3261
The SIP protocol version in the request is not supported.
- 513Message Too LargeRFC 3261
The message is longer than the server can process.
- 580Precondition FailureRFC 3312
The required preconditions for the session, such as resource reservation, could not be met.
6xx Global failure
- 600Busy EverywhereRFC 3261
The called party is busy, and no other location will help.
- 603DeclineRFC 3261
The called party, or a server on their behalf, explicitly declined the call.
- 604Does Not Exist AnywhereRFC 3261
The called user does not exist anywhere.
- 606Not AcceptableRFC 3261
The called party accepts the call but none of the offered session parameters.
- 607UnwantedRFC 8197
The called party marked the call as unwanted, for example as spam.
- 608RejectedRFC 8688
An intermediary, such as an analytics or anti-fraud service, rejected the call.
SIP response code questions
What are SIP response codes?
Which SIP codes are errors?
Is 401 Unauthorized an error?
What is the difference between 4xx and 6xx?
Where do I see the SIP code of a failed call?
Most codes trace back to setup. Check yours.
The four trunk settings and the dial rule, in the menus of common phone systems: SIP trunk configuration.