SIP messages are short technical messages that are exchanged between the devices involved when establishing, controlling, and terminating VoIP telephone connections. They function similarly to status codes on the web and provide information about what is happening with a call—from the initial connection attempt and ringing to successful acceptance or an error.

1xx Informative feedback

Code Description Meaning
100 Trying Processing in progress The target network or device has been reached and the connection is being established in the background. The caller cannot hear a dial tone yet.
180 ringing Call is being signaled The connection has been established to the called party's device. The phone is ringing there and the caller hears a dial tone.
181 Call is Being Forwarded Forwarding active The call is being forwarded to another phone number or extension.
182 Queued In queue The called party is currently unavailable, but the call is being placed in a queue.
183 Session Progress Connection establishment in progress (with interim information) Media data is being transmitted even though the call has not yet been accepted. Typical examples are ring tones, hold music, or automatic announcements.

2xx Successful responses

Code Description Meaning
200 OK "Successful" The call has been successfully connected. Both participants can now talk to each other.
202 Accepted "Accepted" The connection request has been accepted and is being processed. The call can begin shortly.

3xx Redirection Feedback

Code Description Meaning
300 Multiple Choices Multiple destinations possible There are several possible destinations (e.g., different extensions or phone numbers). The caller or device can decide which destination to select.
301 Moved Permanently Permanent forwarding The specified phone number/extension is permanently active at a new location/successor. The new address should be used in future.
302 Moved Temporarily Temporary forwarding The subscriber can currently be reached at a different phone number/extension; this is temporary.
305 Use Proxy Proxy required A proxy server must be used to reach the destination.
380 Alternative Service Alternative service The connection attempt was unsuccessful, but there are alternative routes or destinations.

4xx Error message during application

Code Description Meaning
400 Bad Request Invalid connection The request was incorrectly formatted or could not be processed correctly.
401 Unauthorized Not authorized The login failed. Check your SIP username and password.
402 Payment Required Payment required The connection can only be established after billing or payment.
403 Forbidden Access denied The call could not be delivered because the service is unavailable or permission is missing.
404 Not Found Subscriber not found The specified phone number or extension does not exist.
405 Method not Allowed Method not allowed The SIP method used (e.g., INVITE, REGISTER) is not permitted in this context.
406 Not Acceptable Not acceptable The request could not be fulfilled because conditions (e.g., codec, message format) are not accepted.
407 Proxy Authentication Required Proxy authentication required Authentication with a proxy is required before the connection can be established.
408 Request Timeout Request timeout The server or intermediary did not respond in time.
409 Conflict Conflict The connection cannot be established due to conflicting states.
410 Gone Permanently gone/deleted The resource (e.g., extension/phone number) no longer exists, no forwarding available.
411 Length Required Length specification required A necessary parameter (e.g., content length) is missing from the request.
413 Request Entity Too Large Request data too large The request is larger than allowed.
414 Request-URI too Long URL too long The address/user ID in the request exceeds the supported length.
415 Unsupported Media Type Media type not supported The transmitted media format (e.g., audio codec) is not supported.
416 Range Not Satisfiable / Unsupported URI Scheme Range / URI scheme not suitable Specified values are outside possible limits or schema is not allowed.
420 Bad Extension Extension invalid An optional extension feature in the request is not understood or is not allowed.
480 Temporarily Unavailable Subscriber temporarily unavailable The destination device or extension is currently unavailable (e.g., offline).
481 Call/Transaction Does Not Exist Connection or request does not exist No reference to the request/transaction exists (e.g., in the case of a delayed request).
482 Loop Detected Call/message loop detected The call could not be delivered because a loop was detected in a forwarding.
484 Address Incomplete Address incomplete The specified address (e.g., URI or domain) is incomplete/missing.
485 Ambiguous There are several possible addresses, it is not clear which one to use.  
486 Busy Here The destination device/extension is actively busy (e.g., in another call).  
487 Request Terminated The call was terminated before it was connected.  

5xx error message from the server

Code Description Meaning
500 Internal Server Error Internal server error The server or a gateway has encountered an unexpected error.
501 Not Implemented Not implemented A requested function/method is not supported by the server/gateway.
502 Bad Gateway Error in the gateway server A gateway server has received an invalid response or an error.
503 Service Unavailable Service unavailable The server or destination device is temporarily out of service.
504 Gateway Timeout Gateway timeout The call could not be delivered because the gateway or next server did not respond in time.
505 Version Not Supported Version not supported The SIP protocol version or another scheme is not supported.

6xx Global Responses

Code Description Meaning
600 Busy Everywhere The subscriber is busy at all accessible endpoints (extensions).  
603 Decline The subscriber actively rejects the call (e.g., "no answer" or deliberately rejected).  
604 Does Not Exist Anywhere Does not exist anywhere The dialed phone number or extension does not exist in the network.
606 Not Acceptable Not acceptable The requested connection/media format cannot be accepted for performance or policy reasons.
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Created on: 02. March 2026