IMS SIP Requests and Codes

SIP Requests and Descriptions

In typical VoLTE point of view here is a list of all SIP messages and their meaning. In the rightmost column you can find the RFC number.

SIP Request Description Definition
INVITE Indicates that a client is being invited to participate in a call session RFC 3261
ACK Confirms that the client has received a final response to an INVITE request RFC 3261
BYE Terminates a call; can be sent by either the caller or the called party RFC 3261
CANCEL Cancels any pending request RFC 3261
OPTIONS Queries the capabilities of servers RFC 3261
PRACK Provisional acknowledgement RFC 3262
SUBSCRIBE Subscribes to event notification RFC 3265
NOTIFY Notifies the subscriber of a new Event RFC 3265
PUBLISH Publishes an event to the Server RFC 3903
INFO Sends mid-session information that does not modify the session state RFC 6086
REFER Asks recipient to issue a SIP request (call transfer) RFC 3515
MESSAGE Transports instant messages using SIP RFC 3428
UPDATE Modifies the state of a session without changing the state of the dialog. RFC 3311

SIP Codes and Descriptions

SIP is based around request/response transactions, in a similar manner to the Hypertext Transfer Protocol (HTTP). Each transaction consists of a SIP request (which will be one of several request methods), and at least one response.

These codes are grouped according to their first digit as “provisional”, “success”, “redirection”, “client error”, “server error” or “global failure” codes, corresponding to a first digit of 1–6; these are expressed as, for example, “1xx” for provisional responses with a code of 100–199.

SIP Code Description
1xx Provisional Responses
100 Trying
180 Ringing
181 Call is Being Forwarded
181 Queued
183 Session Progress
199 Early Dialog Terminated
2xx Successful Response
200 OK
202 Accepted
204 No Notification
3xx Redirection Response
300 Multiple Choices
301 Moved Permanently
302 Moved Temporarily
305 Use Proxy
380 Alternative Service
4xx Client Failure Response
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
412 Conditional Request Failed
413 Request Entity Too Large
414 Request-URI Too Long
415 Unsupported Media Type
416 Unsupported URI Scheme
417 Unknown Resource-Priority
420 Bad Extension
421 Extension Required
422 Session Interval Too Small
423 Interval Too Brief
428 Use Identity Header
429 Provide Referer Identity
430 Flow Failed
433 Anonymity Disallowed
436 Bad Identity-Info
437 Unsupported Certificate
438 Invalid Identity Header
439 First Hop Lacks Outbound Support
440 Max-Breadth Exceeded
469 Bad Info Package
470 Consent Needed
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
494 Security Agreement Required
5xx Server Failure Response
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
580 Precondition Failure
6xx Global Failure Response
600 Busy Everywhere
603 Decline
604 Does Not Exist Anywhere
606 Not Acceptable


  1. Nice collation of SIP info Prashant

  2. This chaptered collection of VoLTE info/history is incredibly interesting; to the point where I struggled to continue reading despite actively falling asleep! I had to finish it upon waking up, which I did because the content was intriguing enough for me to remember wanting to finish. So I’d like to extend my gratitude for your thorough compilation of this information, thank you kindly sir.

    Though still absolutely understandable as is, I could tell that English is not your first language. I’m not saying that as an insult, but rather to further exemplify just how amazingly impressed I am with this little VoLTE series.

    In fact, I enjoyed it so much that I’d like to extend an offer to review it for grammatical improvements if you are interested. I’m not one with loads of excess time on my hands, but this just seems like such a great overview of recent mobile standards blunders/fixes/improvements. I’d be honored to help, and I certainly cannot do so from a technical perspective! So email me if you are open to such feedback.

    Thanks again for these great posts!

  3. its a great post about LTE.I never seen this type posts to understand easly,

    thanks again and again by heart….

  4. Thank you, great material

  5. Thanks a lot. You are amazing.. 🙂

  6. any one know what this error for?
    onCallTerminated reasonCode=510

  7. Thanks a lot for this great stuff…Pls continue to provide the post on Signalling and voice traffic flow end to end, Call flow.

  8. Very informative, nicely articulated

  9. What will be the reason code for “No SIP session”?

  10. Big Question. Which network error actually causes poor experience?
    Which ones are fine and a redundant network will fix on its own?

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