1xx Provisional Responses
Provisional SIP responses indicate that the request was received and call/session processing is still ongoing.
IMS SIP response codes
IMS SIP Responses
SIP response codes are used in IMS networks to report progress, success, redirection, authentication challenge, rejection, timeout, media negotiation failure, server error, or global failure. In VoLTE, VoNR, and IMS troubleshooting, responses such as 100 Trying, 180 Ringing, 183 Session Progress, 200 OK, 401 Unauthorized, 403 Forbidden, 486 Busy Here, 487 Request Terminated, and 488 Not Acceptable Here are essential for registration and call-flow analysis.
Use this page to browse IMS SIP responses by status-code class and understand where each response appears in IMS signalling.
Grouped SIP response sections
2xx Success Responses
Success responses indicate that the SIP request was accepted or completed successfully.
3xx Redirection Responses
Redirection responses indicate that the request may need to be redirected to another contact, service, or destination.
- 300 Multiple Choices
- 301 Moved Permanently
- 302 Moved Temporarily
- 305 Use Proxy
- 380 Alternative Service
4xx Client Failure Responses
Client failure responses usually indicate request errors, authentication challenges, policy rejection, timeout, busy user, unsupported media, or unacceptable SDP/capability negotiation.
- 400 Bad Request
- 401 Unauthorized
- 402 Payment Required
- 403 Forbidden
- 404 Not Found
- 405 Method Not Allowed
- 406 Not Acceptable
- 407 Proxy Authentication Required
- 408 Request Timeout
- 410 Gone
- 413 Request Entity Too Large
- 414 Request-URI Too Long
- 415 Unsupported Media Type
- 416 Unsupported URI Scheme
- 420 Bad Extension
- 421 Extension Required
- 423 Interval Too Brief
- 480 Temporarily Unavailable
- 481 Call/Transaction Does Not Exist
- 482 Loop Detected
- 483 Too Many Hops
- 484 Address Incomplete
- 485 Ambiguous
- 486 Busy Here
- 487 Request Terminated
- 488 Not Acceptable Here
- 489 Bad Event
- 491 Request Pending
- 493 Undecipherable
5xx Server Failure Responses
Server failure responses indicate temporary or internal server-side problems during IMS request processing.
- 500 Server Internal Error
- 501 Not Implemented
- 502 Bad Gateway
- 503 Service Unavailable
- 504 Server Time-out
- 505 Version Not Supported
- 513 Message Too Large
6xx Global Failure Responses
Global failure responses indicate that the request cannot be completed by any server or endpoint under the current conditions.
IMS Registration Responses
Common SIP responses seen during IMS registration and authentication.
- 100 Trying
- 401 Unauthorized
- 403 Forbidden
- 408 Request Timeout
- 423 Interval Too Brief
- 200 OK
Call Setup Responses
Common SIP responses seen during VoLTE, VoNR, and IMS call establishment.
Media / SDP Failure Responses
Responses commonly used when SDP, codec, media type, preconditions, or session parameters are not acceptable.
Most used SIP responses
100 Trying request received and processing started 180 Ringing terminating side is alerting 183 Session Progress early session progress, often with SDP 200 OK request succeeded 202 Accepted request accepted for later processing 302 Moved Temporarily temporary redirection 380 Alternative Service alternative service suggested 401 Unauthorized authentication challenge 403 Forbidden service or policy rejection 404 Not Found user or resource not found 407 Proxy Authentication Required proxy authentication challenge 408 Request Timeout no timely response 415 Unsupported Media Type unsupported body/media type 480 Temporarily Unavailable user temporarily unreachable 486 Busy Here target busy 487 Request Terminated INVITE cancelled 488 Not Acceptable Here SDP/media not acceptable 500 Server Internal Error server-side error 503 Service Unavailable service temporarily unavailable 603 Decline global call decline
Common SIP response flows
IMS Registration
REGISTER -> 401 Unauthorized -> REGISTER with Authorization -> 200 OK
Normal Call Setup
INVITE -> 100 Trying -> 180 Ringing / 183 Session Progress -> 200 OK -> ACK
Early Media
INVITE -> 183 Session Progress -> PRACK -> 200 OK for PRACK
Busy Call
INVITE -> 100 Trying -> 486 Busy Here -> ACK
Cancelled Call
INVITE -> CANCEL -> 200 OK for CANCEL -> 487 Request Terminated -> ACK
SDP / Codec Failure
INVITE or UPDATE -> 488 Not Acceptable Here / 415 Unsupported Media Type
Server Failure
INVITE / REGISTER -> 500 Server Internal Error / 503 Service Unavailable
FAQ
What are SIP responses in IMS?
SIP responses report progress, success, redirection, rejection, timeout, authentication challenge, or failure for SIP requests used in IMS registration, call setup, session updates, and release.
What do SIP 1xx responses mean?
SIP 1xx responses are provisional responses. They mean the request was received and processing is still ongoing before a final success or failure response is sent.
What is 183 Session Progress used for?
183 Session Progress indicates early session progress and often carries SDP for early media, preconditions, or media negotiation before final answer.
What does 200 OK mean in SIP?
200 OK means the SIP request succeeded. In IMS it appears in registration, PRACK handling, UPDATE handling, INVITE answer, BYE release, and many service procedures.
Why does IMS registration receive 401 Unauthorized?
401 Unauthorized is normally used as an authentication challenge during IMS registration. The UE responds by sending REGISTER again with Authorization credentials.
What is the difference between 486 Busy Here and 603 Decline?
486 Busy Here means the target is busy at the contacted destination. 603 Decline is a global refusal that tells the sender not to keep trying alternate locations for that user.
What does 487 Request Terminated mean?
487 Request Terminated usually appears when a pending INVITE is cancelled before final answer, commonly after a CANCEL transaction.
What does 488 Not Acceptable Here mean?
488 Not Acceptable Here usually points to SDP, codec, media, precondition, or capability negotiation problems in the current session context.
What is the difference between 4xx and 5xx SIP responses?
4xx responses usually indicate request, authentication, policy, user, or media negotiation problems. 5xx responses indicate server-side, proxy, backend, overload, or temporary service problems.
Which SIP responses are most important in VoLTE and VoNR?
The most important responses include 100 Trying, 180 Ringing, 183 Session Progress, 200 OK, 401 Unauthorized, 403 Forbidden, 486 Busy Here, 487 Request Terminated, 488 Not Acceptable Here, 500 Server Internal Error, and 503 Service Unavailable.