VoIP & SIP error codes

VoIP & SIP faults are reported as a SIP response code. Find yours below, or search for it directly.

Client error (4xx)

• SIP 403ForbiddenYour credentials were accepted but the action was not allowed. A 403 means the system knows who you…
• SIP 480Temporarily UnavailableThe extension exists but nothing is available to ring. On a hosted system this nearly always means the…
• SIP 401UnauthorizedA challenge for credentials, not a failure. One 401 followed by a successful registration is how SIP authentication…
• SIP 404Not FoundThe number or extension you dialled does not exist on the system that answered. On a business phone…
• SIP 408Request TimeoutNo reply arrived before the timer expired. A 408 is a statement that signalling went unanswered, so it…
• SIP 488Not Acceptable HereThe call was acceptable but the audio arrangement was not. A 488 is a media negotiation failure, so…
• SIP 481Call/Transaction Does Not ExistA message arrived for a call the receiving system no longer recognises. It is the signature of SIP…
• SIP 407Proxy Authentication RequiredA proxy in the call path wants credentials before it will route the call. Like a 401 it…
• SIP 486Busy HereThe person you called is busy on this device. It is a healthy response rather than a fault,…
• SIP 487Request TerminatedThe call attempt was cancelled before anyone answered, usually because the caller hung up or because another phone…
• SIP 483Too Many HopsThe call was passed along so many times that a safety counter ran out, which almost always means…
• SIP 484Address IncompleteThe number looks incomplete to the system handling it. On a business phone system that normally means the…
• SIP 400Bad RequestThe request was malformed. Very little SIP equipment sends genuinely broken messages, so when a 400 appears the…
• SIP 415Unsupported Media TypeThe receiving system does not support the format of the message body it was sent. In voice terms…
• SIP 433Anonymity DisallowedThe recipient refuses calls that withhold their number. If your outbound calls are being rejected with this, the…
• SIP 423Interval Too BriefThe handset asked to register for a shorter period than the server allows. It is a straightforward configuration…
• SIP 491Request PendingBoth ends tried to renegotiate the same call at once, so one was asked to wait. It is…
• SIP 405Method Not AllowedThe far end recognises the address but will not accept that kind of request. In practice this shows…
• SIP 420Bad ExtensionYour device insisted on a SIP extension the far end does not understand. It is a negotiation mismatch…
• SIP 482Loop DetectedA server spotted that it had already handled this call and stopped it going round again. Like a…
• SIP 410GoneThe number or extension existed once and has deliberately been withdrawn. It is a more informative version of…
• SIP 421Extension RequiredThe mirror image of a 420: the server needs an extension your device did not offer, and names…

Server error (5xx)

• SIP 503Service UnavailableA server in the path is temporarily unable to handle the call. On a hosted system this most…
• SIP 500Server Internal ErrorA server hit an unexpected internal condition. It is a genuine fault rather than a policy decision, and…
• SIP 504Server Time-outA server did reply, to tell you it could not get an answer from something further along. Unlike…
• SIP 502Bad GatewayA proxy passed your call on and got back something it could not use. It points at the…
• SIP 501Not ImplementedThe server does not support the thing you asked it to do at all. Unlike a 405, which…

Global failure (6xx)

• SIP 603DeclineThe call was actively declined rather than failing. Someone or something chose to reject it, which on outbound…
• SIP 607UnwantedThe recipient marked the call as unwanted. For a legitimate business making outbound calls this is a reputation…
• SIP 608RejectedA network in the middle blocked the call before it reached the recipient, judging it unwanted. Unlike a…
• SIP 604Does Not Exist AnywhereThe number does not exist anywhere, not just on the system that answered. That usually means the number…
• SIP 600Busy EverywhereThe person you called is busy on every device they have. Unlike a 486, which refuses at one…
• SIP 606Not AcceptableThe session as described is not acceptable anywhere for that user. It is the global counterpart of a…

Redirection (3xx)

• SIP 302Moved TemporarilyThe call is being sent somewhere else, and the response says where. This is how call forwarding works,…
• SIP 380Alternative ServiceThe call did not succeed as asked, but an alternative is offered. In UK deployments this most often…
• SIP 301Moved PermanentlyThe address has moved for good, and the response gives the new one. Unlike a 302, which is…

Provisional (1xx)

• SIP 100TryingNot an error. It confirms your request was received and is being worked on, which stops the sending…
• SIP 180RingingNot an error. The destination is ringing. It is useful diagnostically because it proves the call reached a…
• SIP 183Session ProgressAudio is being sent before the call is answered. This is how you hear a network announcement, a…

Success (2xx)

• SIP 200OKNot an error. It is the success response, whether that is a call being answered or a registration…

Every other SIP response code

These are the remaining codes in the official registry. They are extension or rarely-used codes that we have not written up, so they are listed here for reference with the document that defines each one.

Source. Code numbers and official names are taken from the IANA SIP Parameters registry, with each code's definition linked to the RFC that specifies it. Causes and fixes are our own, from supporting UK business phone systems.