SIP Error Response Codes

SIP response codes help identify what happened during registration or call setup. Use this guide to interpret common responses when troubleshooting SIP devices, PBX systems, and trunks.

Common 2talk troubleshooting codes

401 Unauthorized

A 401 response is normal during SIP authentication. 2talk challenges the device, and the device should reply with valid digest authentication details. If registration continues to fail after the challenge, check the username, auth ID, password, and domain.

403 Forbidden

403 means the request was understood but refused. Common causes include incorrect caller ID presentation, invalid credentials, permission issues, or no available account credit for the attempted call.

407 Proxy Authentication Required

The proxy requires authentication. Confirm that the device is sending the correct SIP username, auth ID, and password.

487 Request Terminated

The call was canceled before it was answered. This commonly appears when the caller hangs up during call setup.

Response code groups

1xx Informational

• 100 Trying: The request was received and is being processed.
• 180 Ringing: The destination is being alerted.
• 183 Session Progress: Early media or setup information is available.

2xx Success

• 200 OK: The request succeeded.

4xx Request failures

• 400 Bad Request: The request syntax is malformed.
• 401 Unauthorized: Authentication challenge or failed authentication.
• 403 Forbidden: The request was refused.
• 404 Not Found: The requested user or domain was not found.
• 407 Proxy Authentication Required: Proxy authentication is required.
• 408 Request Timeout: No response was available in time.

5xx Server errors

• 500 Internal Server Error: The server could not complete the request.
• 502 Bad Gateway: A gateway or proxy received an invalid response.
• 503 Service Unavailable: The server is unavailable or overloaded.

6xx Global failures

• 600 Busy Everywhere: All destinations are busy.
• 603 Decline: The destination declined the call.
• 604 Does Not Exist Anywhere: The requested user does not exist.
• 606 Not Acceptable: The session description was not acceptable.
• 607 Unwanted: The called party rejected the call as unwanted.

SDP and media details

SIP signaling sets up the call, while SDP describes media details such as RTP port, codec list, and DTMF method. If calls connect but audio or DTMF fails, compare the SDP offer and answer.

• m=audio: Defines the RTP port and codec payload list.
• 0: PCMU.
• 8: PCMA.
• 18: G.729.
• 101: telephone-event, often used for RFC 2833 DTMF.