What are SIP response codes?
SIP response codes are status messages used in the SIP protocol to indicate the outcome of a SIP request — such as registering a device or placing a call. These codes help identify whether communication is proceeding normally or if something is going wrong.
Understanding these codes can help you troubleshoot issues faster and ensure your VoIP setup runs smoothly.
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Response codes list
1xx—Informational Responses
These responses indicate that a request is being processed, but no final answer is available yet.
Code | Meaning | What it means | Possible cause / action |
100 | Trying | The request is being processed. | No action needed. This is a normal part of SIP call setup. |
180 | Ringing | The destination device is ringing. | Normal signaling. The phone is alerting the user. |
181 | Call Is Being Forwarded | The call is being forwarded to another destination. | User has set up call forwarding. Check settings if unexpected. |
182 | Queued | The call is queued and will be processed. | Often used in call center setups. |
183 | Session Progress | Provides early media (e.g., ringback tone) before call is answered. | Can be normal; some carriers/devices use this to deliver call progress tones. |
2xx—Successful Responses
These indicate that the SIP request was successfully received, understood, and accepted.
Code | Meaning | What it means | Possible cause / action |
200 | OK | The request was successful. | Normal behavior; indicates success in registration, call setup, etc. |
202 | Accepted | Request accepted for processing, but not completed. | Rare in SIP; sometimes used for deferred actions. |
3xx—Redirection Responses
These codes indicate that the request should be redirected to a different endpoint. They are not necessarily errors, but can influence call behavior and routing.
Code | Meaning | What it means | Possible cause / action |
300 | Multiple Choices | The address returned has multiple choices for the user or client to select from. | Rarely used; could mean call forwarding is configured with multiple targets. |
301 | Moved Permanently | The user has moved to a new address, and future requests should go there. | May indicate permanent call forwarding or device reassignment. |
302 | Moved Temporarily | The user is temporarily available at a different address. | Temporary redirection due to call forwarding or registration at another endpoint. |
305 | Use Proxy | The request must be sent through a specific proxy server. | Check if SIP client supports and is correctly configured to use the indicated proxy. |
380 | Alternative Service | The call failed, but alternative services are available. | Often related to call features not being supported; user might need to use a different service or codec. |
4xx—Client Failure Responses
These indicate that the request is invalid or cannot be fulfilled due to an issue on the client side (e.g., device misconfiguration, authentication failure, or unsupported behavior).
Code | Meaning | What it means | Possible cause / action |
400 | Bad Request | Malformed request syntax or parameters. | Check for typos or formatting errors in the SIP settings. |
401 | Unauthorized | Authentication is required (used by registrars). | Ensure correct SIP credentials; proxies should use 407 instead. |
402 | Payment Required | Reserved for future use. | Not applicable. |
403 | Forbidden | The server refuses the request. | Incorrect credentials or blocked account/IP. |
404 | Not Found | The user or extension does not exist. | Verify the number, extension, or device registration. |
405 | Method Not Allowed | The SIP method is not supported by the server. | Check compatibility with your SIP client. |
406 | Not Acceptable | The resource is not acceptable according to the Accept headers. | May require changing codecs or settings. |
407 | Proxy Authentication Required | The proxy requires authentication. | Double-check SIP username and password. |
408 | Request Timeout | No response from the destination. | Device may be offline or not registered. |
409 | Conflict | Conflict with the current state of the resource. | Rarely used; possibly duplicate registrations. |
410 | Gone | The user existed once but is no longer available. | Account or extension may have been removed. |
412 | Conditional Request Failed | A precondition for the request wasn't met. | Usually relates to SIP headers — check advanced config. |
413 | Request Entity Too Large | The request body is too large to process. | Reduce SIP message size or packet payload. |
414 | Request-URI Too Long | The URI in the request is too long. | Invalid SIP request; contact your provider or IT admin. |
415 | Unsupported Media Type | The media format (e.g., codec) is not supported. | Adjust your codec settings. |
416 | Unsupported URI Scheme | The URI scheme is not recognized. | Ensure you're using a standard SIP URI (e.g., sip:user@domain). |
417 | Unknown Resource-Priority | Resource priority header is unknown. | Not commonly used — check with your provider. |
420 | Bad Extension | An unsupported SIP protocol extension was requested. | Disable advanced SIP extensions in your device. |
421 | Extension Required | Server requires a specific SIP extension to process the request. | Check device for missing features or compatibility. |
422 | Session Interval Too Small | Session timer is too short. | Increase session interval (e.g., >90 seconds). |
423 | Interval Too Brief | Requested interval is shorter than the minimum allowed. | Adjust refresh intervals in your device. |
424 | Bad Location Information | The location info provided is invalid. | Not typically relevant for standard VoIP setups. |
428 | Use Identity Header | The request lacks an Identity header. | May require advanced SIP identity settings. |
429 | Provide Referrer Identity | Missing or invalid identity in a referral. | Rare in basic VoIP setups; advanced feature. |
433 | Anonymity Disallowed | Anonymous requests are not permitted. | Disable anonymity features in your device if used. |
436 | Bad Identity-Info | The Identity-Info header is invalid. | SIP identity settings may need correction. |
437 | Unsupported Certificate | The certificate provided is not supported. | May relate to encrypted SIP (TLS); verify certificate settings. |
438 | Invalid Identity Header | Identity header content is not valid. | Check SIP Identity settings or contact your provider. |
480 | Temporarily Unavailable | The destination is offline or unreachable. | Device not registered, turned off, or on Do Not Disturb. |
481 | Call/Transaction Does Not Exist | The session or call ID doesn't match an active transaction. | Often caused by delays or dropped packets. |
482 | Loop Detected | SIP message is caught in a routing loop. | Check forwarding rules or PBX config. |
483 | Too Many Hops | SIP request exceeded the maximum hop count. | Routing loop or too many forwarding steps. |
484 | Address Incomplete | The request URI is missing required elements. | Check the number being dialed; may be incomplete. |
485 | Ambiguous | The address provided matches multiple resources. | Use a more specific or unique SIP address. |
486 | Busy Here | The recipient is on another call or unavailable. | Either busy on another line or using Do Not Disturb. |
487 | Request Terminated | The request was canceled before completion. | The caller may have hung up or the transaction timed out. |
488 | Not Acceptable Here | Media parameters are not acceptable at the destination. | Mismatch in codecs or media negotiation. |
489 | Bad Event | The event header or package is not understood. | Event notifications may be misconfigured. |
491 | Request Pending | The request overlaps with an ongoing transaction. | Wait before retrying. |
493 | Undecipherable | Encrypted message cannot be decrypted. | Possible issue with S/MIME or TLS keys. |
494 | Security Agreement Required | Security negotiation is required but missing or failed. | May need to enable secure SIP (SIPS or TLS). |
5xx—Server Failure Responses
These mean the server failed to fulfill a valid request.
Code | Meaning | What it means | Possible cause / action |
500 | Server Internal Error | Generic server-side error. | Temporary issue on the SIP server. Retry later or contact support. |
501 | Not Implemented | SIP method is not supported by the server. | The server doesn't recognize the request type. Check compatibility. |
502 | Bad Gateway | Received invalid response from upstream server. | May indicate carrier issues or misconfigured proxies. |
503 | Service Unavailable | The server is temporarily unable to handle the request. | Often a sign of overload or maintenance. Try again later. |
504 | Server Time-out | Server did not respond in time. | Network delays, firewall issues, or problems on the upstream side. |
505 | Version Not Supported | SIP version used is not supported by the server. | Extremely rare. Check if the SIP stack is outdated or misconfigured. |
513 | Message Too Large | The request was too large for the server to handle. | May be due to large SDP or too much header info. Check configuration. |
580 | Precondition Failure | Precondition in the request message wasn’t met. | Can relate to media negotiation. Troubleshoot SDP or device compatibility. |
6xx—Global Failure Responses
These indicate that the request failed at all possible destinations.
Code | Meaning | What it means | Possible cause / action |
600 | Busy Everywhere | All destinations are busy. | All forwarding targets or user endpoints are in use. |
603 | Decline | The destination declined the call. | User actively rejected the call or auto-decline settings are enabled. |
604 | Does Not Exist Anywhere | The user doesn't exist at any known location. | May indicate an unregistered number or misdial. |
606 | Not Acceptable | The request was understood but not acceptable for the user. | Codec, encryption, or media incompatibility. Review supported settings on both sides. |
Related Pages
SIP (Session Initiation Protocol)
SIP (Session Initiation Protocol)