Session Initiation Protocol (SIP) uses response codes to indicate the status of communication requests in VoIP systems. These codes are three-digit numbers grouped into six categories, each representing a different type of response.
🔢 Categories of SIP Response Codes
| Code Range | Category | Description |
|---|---|---|
| 1xx | Provisional Responses | Request received, continuing process |
| 2xx | Successful Responses | Action successfully received, understood, accepted |
| 3xx | Redirection Responses | Further action needs to be taken to complete the request |
| 4xx | Client Error Responses | Request contains bad syntax or cannot be fulfilled |
| 5xx | Server Error Responses | Server failed to fulfill a valid request |
| 6xx | Global Failure Responses | Request cannot be fulfilled at any server |
📘 Detailed SIP Response Codes
1xx – Provisional Responses
These indicate that the request has been received and processing is continuing.
- 100 Trying: Request received, continuing process.
- 180 Ringing: Destination user agent received INVITE, alerting user.
- 181 Call Is Being Forwarded: Call is being forwarded to another destination.
- 182 Queued: Destination temporarily unavailable; call queued.
- 183 Session Progress: Provides information about the progress of the call.
2xx – Successful Responses
These codes indicate that the action was successfully received, understood, and accepted.
- 200 OK: Request succeeded.
- 202 Accepted: Request accepted for processing, but not completed.
- 204 No Notification: Request succeeded; no additional information will be sent.
3xx – Redirection Responses
Further action needs to be taken to complete the request.
- 300 Multiple Choices: Multiple options for the resource that the client may follow.
- 301 Moved Permanently: Requested resource has been assigned a new permanent URI.
- 302 Moved Temporarily: Requested resource resides temporarily under a different URI.
- 305 Use Proxy: Must access the requested resource through the proxy given by the Contact field.
- 380 Alternative Service: Call failed; alternatives are detailed in the message body.
4xx – Client Error Responses
The request contains bad syntax or cannot be fulfilled at this server.
- 400 Bad Request: Malformed request syntax.
- 401 Unauthorized: Request requires user authentication.
- 403 Forbidden: Server understood the request but refuses to authorize it.
- 404 Not Found: Server has definitive information that the user does not exist.
- 486 Busy Here: Callee is busy.
5xx – Server Error Responses
The server failed to fulfill an apparently valid request
- 500 Server Internal Error: Server encountered an unexpected condition.
- 501 Not Implemented: Server does not support the functionality required.
- 502 Bad Gateway: Received an invalid response from an upstream server.
- 503 Service Unavailable: Server is currently unable to handle the request.
- 504 Server Time-out: Server did not receive a timely response from an upstream server.
6xx – Global Failure Responses
The request cannot be fulfilled at any server.
- 600 Busy Everywhere: All possible destinations are busy.
- 603 Decline: Destination does not wish to participate in the call.
- 604 Does Not Exist Anywhere: User does not exist anywhere.
- 606 Not Acceptable: User’s agent was contacted successfully but some aspects of the session description were not acceptable.
🖼️ Visual Representation
This diagram illustrates the categorization of SIP response codes.
🔍 Conclusion
Understanding SIP response codes is crucial for diagnosing and troubleshooting VoIP communications. These codes provide insight into the status of SIP requests and help in identifying issues in call setups.
For a comprehensive list and more detailed explanations, refer to the