SIP Error Response Categories

SIP response codes are grouped into six categories:

  • 1xx: Informational
    Temporary responses indicating progress in the request.
    Example: 180 Ringing – The call is ringing.

  • 2xx: Success
    Indicates the request was successful.
    Example: 200 OK – The request was completed successfully.

  • 3xx: Redirection
    Further action is needed to complete the request.
    Example: 302 Moved Temporarily – The user is temporarily available at a different address.

  • 4xx: Client Errors
    Issues caused by the client.
    Example: 404 Not Found – The requested user does not exist.

  • 5xx: Server Errors
    Errors on the server-side.
    Example: 503 Service Unavailable – The server is temporarily unable to process the request.

  • 6xx: Global Errors
    Issues affecting the entire system.
    Example: 603 Decline – The user does not wish to answer the call.

Common SIP Error Codes and Their Meaning

4xx: Client Error Responses

  1. 400 Bad Request

    • Cause: The request is malformed or contains invalid syntax.
    • Resolution: Verify the SIP headers, formatting, and parameters.

  2. 401 Unauthorized

    • Cause: Authentication is required, but credentials are missing or incorrect.
    • Resolution: Check and update SIP user credentials in your VoIP device or PBX.

  3. 403 Forbidden

    • Cause: The server understood the request but refuses to process it.
    • Resolution: Verify that the account has the necessary permissions and the IP is not blocked.

  4. 404 Not Found

    • Cause: The user or resource is unavailable at the specified address.
    • Resolution: Confirm the SIP address or phone number is correct.

  5. 408 Request Timeout

    • Cause: The server did not receive a timely response.
    • Resolution: Check network connectivity and server responsiveness.

  6. 480 Temporarily Unavailable

    • Cause: The user is currently unavailable.
    • Resolution: Retry later or check the user’s device status.

5xx: Server Error Responses

  1. 500 Internal Server Error

    • Cause: An unexpected server issue occurred.
    • Resolution: Check server logs and restart services if needed.

  2. 503 Service Unavailable

    • Cause: The server is temporarily out of service.
    • Resolution: Verify server health, resource availability, and connectivity.

  3. 504 Server Time-out

    • Cause: The server took too long to respond.
    • Resolution: Investigate network latency or server performance.

6xx: Global Error Responses

  1. 600 Busy Everywhere

    • Cause: The user is busy on all devices.
    • Resolution: Inform the caller and retry later.

  2. 603 Decline

    • Cause: The user has declined the call.
    • Resolution: Contact the user to ensure availability.

  3. 606 Not Acceptable

    • Cause: The requested media type or parameters are not supported.
    • Resolution: Adjust the media settings in the SIP client or PBX.

Diagnosing SIP Errors

To troubleshoot SIP errors effectively:

  1. Use SIP Debugging Tools: Tools like Wireshark or SIP logs in your PBX can help analyze SIP traffic.
  2. Check Configuration: Verify the SIP trunk, device settings, and user credentials.
  3. Test Network Connectivity: Ensure there are no firewall or NAT issues blocking SIP communication.
  4. Monitor Call Flow: Use tools like SIP ladder diagrams to trace the problem.

Preventing SIP Errors

  1. Ensure Proper Configuration: Double-check your SIP settings for accuracy.
  2. Update Software: Use the latest firmware or software versions for SIP devices.
  3. Optimize Network Settings: Configure QoS and avoid SIP ALG (Application Layer Gateway) on firewalls.
  4. Monitor System Health: Regularly check server resources and connection stability.