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Understanding SIP Response Codes (Status Messages)

Session Initiation Protocol (SIP) is the standard technology used to establish, manage, and end calls over VoIP networks. SIP response codes are three-digit numeric messages sent during a VoIP call to communicate the status of a call between the calling party (UAC—User Agent Client) and the receiving party (UAS—User Agent Server).

Knowing these codes helps you quickly understand the status of your calls, troubleshoot problems, and manage VoIP communications effectively.

Categories of SIP Response Codes

SIP codes are organized into six main categories, identified by their first digit:

• 1xx – Informational: Request is in progress.
• 2xx – Success: Request completed successfully.
• 3xx – Redirection: Call redirected to another destination.
• 4xx – Client Error: Issue with the request itself.
• 5xx – Server Error: Issue on the server side.
• 6xx – Global Error: Request rejected globally (no other servers will accept the request).
  

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1xx: Informational Responses

These codes indicate the ongoing status of the call:

• 100 Trying: The request has been received and is being processed.
• 180 Ringing: The user’s phone is ringing or being alerted.
• 181 Call Is Being Forwarded: Your call is redirected to another number.
• 182 Queued: The call is temporarily placed in a queue.
• 183 Session Progress: The call is in progress; the connection is being established.
• 199 Early Dialog Terminated: The call attempt ended before establishing a full connection.

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2xx: Successful Responses

These responses confirm the request was successful:

• 200: The call or action was successful.
• 202 Accepted: Request was understood and accepted, but may not yet be fully processed.
• 204 No Notification: The request succeeded but doesn’t require further notification.

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3xx: Redirection Responses

These codes show your call request has been redirected:

• 300 Multiple Choices: Multiple call routing options available; the caller chooses one.
• 301 Moved Permanently: The requested user has permanently moved to a new address.
• 302 Moved Temporarily: Temporarily forwarded to a different address; the original address is still valid.
• 305 Use Proxy: Calls must be routed through a specific proxy listed in the response.
• 380 Alternative Service: The call failed, but alternative routes are provided in the response.

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4xx: Client Error Responses

These codes indicate an issue with the caller's request:

• 400 Bad Request: Request could not be understood due to bad syntax.
• 401 Unauthorized: Authentication required to complete the call.
• 403 Forbidden: The server understood but refused the request.
• 404 Not Found: The called party’s number does not exist.
• 405 Method Not Allowed: The method used is invalid for the requested address.
• 407 Proxy Authentication Required: Authentication with the proxy is necessary.
• 408 Request Timeout: No timely response was received from the server.
• 415 Unsupported Media Type: Unsupported media type or format.

Additional 4xx codes you might encounter:

• 402 Payment Required
• 410 Gone
• 413 Request Entity Too Large
• 416 Unsupported URI Scheme
• 420 Bad Extension
• 480 Temporarily Unavailable
• 486 Busy Here: Called party line is busy.
• 487 Request Terminated: Call ended before connection.
• 488 Not Acceptable Here: Request or media type not acceptable.

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5xx: Server Error Responses

These codes indicate problems with the server processing the call:

• 500 Server Internal Error: General unexpected server issue.
• 501 Not Implemented: The server lacks functionality needed for the request.
• 502 Bad Gateway: Invalid response from another server in the network.
• 503 Service Unavailable: Server temporarily overloaded or down for maintenance.
• 504 Server Timeout: No timely response from upstream server.
• 513 Message Too Large: Message exceeds the server’s capabilities.

Additional 5xx codes:

• 505 Version Not Supported
• 555 Push Notification Service Not Supported
• 580 Precondition Failure

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6xx: Global Failure Responses

These responses mean the call cannot proceed to any destination:

• 600 Busy Everywhere: All possible destinations are busy.
• 603 Decline: The call was declined by the recipient.
• 604 Does Not Exist Anywhere: No destination could be found.
• 606 Not Acceptable: The requested call parameters are not supported by the recipient.

Additional 6xx codes:

• 607 Unwanted
• 608 Rejected

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Why Understanding SIP Codes Matters

• Quickly identify call issues.
• Resolve VoIP problems faster.
• Optimize call routing and handling.
• Reduce downtime and improve call quality.

By familiarizing yourself with these SIP codes, you can efficiently troubleshoot issues, clearly communicate with your support team, and maintain smooth operations.

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If you have additional questions about SIP codes or need assistance, please contact our live support or refer to our detailed guides in the IllyVoIP Help Center.

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