SIP Response Codes (also known as Cause Values) are returned by the network or destination system to indicate the outcome of a call attempt. These codes can help identify why an outbound call may not have connected successfully.
The table below outlines some of the response codes that may appear against a call, along with a brief explanation of what they mean.
| SIP Code | Name | Short Description |
| 200 | OK. | The call was successfully connected and answered. |
| 300 | Multiple Choice. | The destination has multiple possible endpoints. |
| 400 | Bad request. | The call request could not be processed due to an invalid format. |
| 401 | Unauthorised. | Authentication is required to place the call. |
| 402 | Payment required. | Reserved for future use. |
| 403 | Forbidden. | The call was rejected by the destination network. |
| 404 | Not found. (Unassigned number) | The number could not be found or is not in service. |
| 405 | Method not allowed. | The request type is not allowed by the destination. |
| 406 | Not acceptable. | The destination cannot support the requested media or format. |
| 407 | Proxy authentication requested. | Authentication is required by the network proxy. |
| 408 | Request timeout. | The destination did not respond in time. |
| 409 | Conflict. | User already registered. |
| 410 | Gone. (Unassigned number) | The number previously existed but is no longer active. |
| 411 | Length required. | The request was rejected because it did not include a required message length. |
| 412 | Conditional request failed. | The request could not be completed because certain conditions included in the request were not met. |
| 413 | Request entity too long. | The request message is larger than the server can process. |
| 414 | Request URI too long. | The requested address is too long to process. |
| 415 | Unsupported media type. | The request contains a media format that is not supported. |
| 416 | Unsupported URI scheme. | The request uses a URI scheme that is not supported. |
| 417 | Unknown resource - priority. | There was a resource-priority option tag, but no Resource-Priority header. |
| 418 | Bad extension. | The request used a SIP feature or extension that the server does not understand or support. |
| 421 | Extension required. | A required SIP extension is missing from the request. |
| 422 | Session interval too small. | The request specified a session duration that is shorter than the minimum allowed by the server. |
| 423 | Interval too brief. | The registration interval is too short. |
| 424 | Bad location information. | The location details included in the request were invalid, incomplete, or incorrectly formatted. |
| 428 | Use identity header. | The server requires the request to include identity verification information, which was not provided. |
| 429 | Provide referrer identity. | The request requires valid referral identity information, but this was missing or invalid. |
| 430 | Flow failed. | The network path used to reach the receiving device has failed. Other available paths may still work. |
| 433 | Anonymity disallowed. | The request was rejected because anonymous requests are not permitted by the server. |
| 436 | Bad identity - info. | The identity information included in the request could not be accessed or verified by the server. |
| 437 | Unsupported certificate. | The certificate used to verify the request could not be validated by the server. |
| 438 | Invalid identity header. | The server was able to find the certificate referenced in the request, but the identity signature could not be verified. |
| 439 | First hop lacks outbound support. | The first network device handling the request does not support the required outbound connection feature. |
| 440 | Max-Breadth Exceeded. | The request could not be sent to all potential destinations because a system limit on simultaneous routing was exceeded. |
| 469 | Bad info package. | The request contained an information package that the receiving system does not support. |
| 470 | Consent needed. | The request was rejected because the sender does not have permission from the recipient to send this request. |
| 480 | Temporarily unavailable. | The destination is temporarily unreachable. |
| 481 | Call/Transaction does not exist. | The call reference is no longer valid |
| 482 | Loop detected. | The call routing created a loop in the network. |
| 483 | Too many hops. | The call passed through too many network routes. |
| 484 | Address incomplete. (Invalid number) | The dialled number is incomplete. |
| 485 | Address ambiguous. (Unassigned number) | The dialled number matches multiple destinations. |
| 486 | Busy here. | The destination line is busy. |
| 487 | Request cancelled. | The call was cancelled before it connected. |
| 488 | Not acceptable here. | The destination cannot support the call parameters. |
| 489 | Bad event. | The request referenced an event type that the server does not recognise or support. |
| 491 | Request pending. | Another request is already being processed. |
| 493 | Undecipherable. | The request could not be processed due to encryption or formatting issues. |
| 494 | Security agreement required. | The server requires a specific security method to be used before the request can be processed. |
| 500 | Internal server error. | The network encountered an unexpected error while processing the call. |
| 501 | Not implemented. | The network does not support the requested call method. |
| 502 | Bad gateway. | The network received an invalid response from another server. |
| 503 | Service unavailable. | The service is temporarily unavailable or overloaded. |
| 504 | Gateway timeout. | The network did not receive a response from another server in time. |
| 505 | Version not implemented. | The SIP version used in the request is not supported. |
| 513 | Message too large. | The request message exceeded the maximum allowed size. |
| 580 | Precondition failed. | The server is unable or unwilling to meet some constaints specified in the offer. |
| 600 | Busy everywhere. | All possible destinations are busy. |
| 603 | Decline. | The call was actively declined by the recipient. |
| 604 | Does not exist anywhere. (Unassigned number) | The number does not exist. |
| 606 | Not acceptable. | The destination cannot accept the requested call parameters. |