Call progress before a final answer.
IMS SIP Response Reference
IMS SIP Response Codes
IMS SIP response codes show call progress, success, redirection, authentication challenges, and failure conditions during IMS registration and session signaling. This page is the main reference hub for SIP responses used in VoLTE, VoNR, and IMS troubleshooting.
How SIP responses work
SIP responses use three-digit status codes with reason phrases to show what happened after a request was received. The first digit gives the fastest practical answer: progress, success, redirection, client-side issue, server-side issue, or global failure.
- 1xx = provisional
- 2xx = success
- 3xx = redirection
- 4xx = client, request, authentication, or user-side issues
- 5xx = server, network, or backend issues
- 6xx = global failures
Response family overview
SIP responses are grouped by status-code class so engineers can quickly separate call progress, success, redirection, authentication issues, user reachability problems, and server-side failures.
Request completed successfully.
Alternate destination or service should be used.
Request, authentication, policy, or user-side issues.
Server, proxy, or backend problem.
Definitive failure for that user everywhere.
Featured response pages
These are the SIP response codes most commonly seen in IMS registration, VoLTE call setup, early media handling, call rejection, and troubleshooting traces.
100 Trying
Shows that the request has entered processing.
call setup 180 RingingIndicates that the called side is alerting.
call setup 183 Session ProgressUsed for early session progress and often early media.
early media 200 OKFinal success response used across registration and session procedures.
success 401 UnauthorizedAuthentication challenge seen heavily during IMS registration.
registration 403 ForbiddenAuthorization or policy rejection.
failure 408 Request TimeoutTransaction did not complete before timeout.
failure 480 Temporarily UnavailableTarget is known but not currently available.
failure 486 Busy HereCalled side is busy at the current destination.
failure 487 Request TerminatedTypical final response after CANCEL or aborted setup.
release 488 Not Acceptable HereOffer, media, or SDP details are not acceptable.
media 500 Server Internal ErrorGeneral server-side processing failure.
server 503 Service UnavailableTemporary overload, outage, or service-side issue.
server 603 DeclineGlobal refusal that blocks retry against alternate destinations.
failureFull response catalog
1xx Provisional responses
These responses show progress while the request is still active and before any final success or failure answer is returned.
2xx Success responses
These responses confirm that the request completed successfully from the SIP point of view.
3xx Redirection responses
These responses tell the sender to try another route, destination, or service treatment.
4xx Client failure responses
These responses usually point to request formatting, authentication, policy, addressing, state, user availability, or media-negotiation issues.
5xx Server failure responses
These responses normally indicate a problem in the server, proxy, service platform, or backend path rather than the user endpoint itself.
6xx Global failures
These responses indicate a definitive failure that should not be retried against alternate user locations.
Responses by IMS procedure
Many users search for SIP responses in the context of registration, call setup, PRACK handling, session modification, or release. Use the groups below to jump directly to the responses typically seen in each procedure.
IMS registration
Call setup and alerting
Early media / reliable provisional handling
Call cancellation / pre-answer release
Reachability / user availability failures
Session and SDP/media failures
Server and network-side failures
Common SIP Response Codes in IMS Troubleshooting
How to read a SIP response page
Each child response page should help you move from the code itself into the exact procedure context and the most useful inspection points in a trace.
- Purpose
- Where it appears in call flow
- Common trigger conditions
- Related request methods
- Whether SDP or message body is common
- Headers or fields to inspect
- Common causes
- Troubleshooting checklist
- Related pages
Related pages
FAQ
What are SIP response codes in IMS?
SIP response codes in IMS are three-digit status codes with reason phrases that show whether a request is progressing, has succeeded, must be redirected, or failed during registration, call setup, session updates, or release.
What is the difference between 1xx, 2xx, 4xx, 5xx, and 6xx in SIP?
1xx means provisional progress, 2xx means success, 4xx means request, user, authentication, policy, or state failure, 5xx means server or backend failure, and 6xx means a global failure that should not be retried elsewhere.
What is the difference between 180 Ringing and 183 Session Progress?
180 Ringing mainly shows alerting, while 183 Session Progress often carries early-session context such as SDP and is commonly associated with early media handling.
Why does IMS registration use 401 Unauthorized?
IMS registration often uses 401 Unauthorized as an authentication challenge so the UE can resend REGISTER with valid credentials and complete the registration exchange securely.
What does 487 Request Terminated mean in a cancelled call?
487 Request Terminated usually means the original INVITE transaction was ended before answer, commonly because CANCEL or equivalent termination logic stopped the pending setup.
What does 488 Not Acceptable Here mean in VoLTE or VoNR?
488 Not Acceptable Here usually points to an offer, codec, media, or SDP mismatch that the receiving side cannot accept in the current session context.
What is the difference between 486 Busy Here and 603 Decline?
486 Busy Here is a destination-specific busy response, while 603 Decline is a global refusal that tells the sender not to keep trying alternate locations for that user.