Registration
SIP methods used when the UE registers with IMS and refreshes or removes registration bindings.
IMS SIP request methods
IMS SIP Requests
SIP request methods are used in IMS networks to start, modify, acknowledge, cancel, release, or query signalling procedures. In VoLTE, VoNR, and IMS, methods such as REGISTER, INVITE, ACK, PRACK, UPDATE, BYE, CANCEL, MESSAGE, OPTIONS, SUBSCRIBE, NOTIFY, REFER, INFO, and PUBLISH appear in registration, call setup, early media, session modification, messaging, supplementary services, and call release flows.
Use this page to browse IMS SIP request methods by procedure category and understand where each method fits in IMS signalling.
Grouped SIP request sections
Call Setup
SIP methods used to establish IMS voice, video, or multimedia sessions.
Reliable Provisional / Early Media
Methods used when provisional responses such as 183 Session Progress are sent reliably.
Session Update
Methods used to change SDP, media direction, preconditions, codec, or session state.
Call Release
Methods used to terminate an established SIP dialog.
Call Cancellation
Methods used to cancel a pending INVITE before final answer.
Messaging
Methods used for pager-mode messaging or IMS service messaging.
Capability / Keepalive
Methods used to query capabilities, test reachability, or support service checks.
Supplementary Services
Methods used for transfer, call control services, and mid-dialog service signalling.
Event Subscription
Methods used for subscription and notification of event packages.
Other / Advanced
Additional SIP methods used in IMS extensions or operator-specific services.
Most used SIP requests
REGISTER IMS registration and registration refresh INVITE Starts a session or re-INVITE ACK Confirms final response to INVITE PRACK Acknowledges reliable provisional response UPDATE Updates session parameters BYE Ends an established dialog CANCEL Cancels pending INVITE MESSAGE Sends pager-mode message OPTIONS Queries capabilities SUBSCRIBE Subscribes to event package NOTIFY Sends event notification REFER Triggers transfer-related service action
Common SIP request flows
Registration
REGISTER -> REGISTER with Authorization
Call Setup
INVITE -> PRACK if reliable provisional response is used -> ACK after 200 OK
Session Update
UPDATE -> response; re-INVITE -> response -> ACK
Call Release
BYE -> response
Call Cancellation
CANCEL -> response, then original INVITE gets 487 Request Terminated
FAQ
What are SIP requests in IMS?
SIP requests are SIP method messages used by IMS endpoints and network functions to start, modify, acknowledge, cancel, release, query, or subscribe to signalling procedures.
Which SIP request starts an IMS call?
INVITE starts IMS session establishment for voice, video, or multimedia calls. ACK later confirms the final successful response to that INVITE.
Which SIP request is used for IMS registration?
REGISTER is used for IMS registration, authentication follow-up, registration refresh, and registration binding updates.
What is the difference between BYE and CANCEL?
BYE terminates an already established SIP dialog. CANCEL stops a pending INVITE transaction before the call has received a final answer.
What is PRACK used for?
PRACK acknowledges reliable provisional responses, commonly reliable 183 Session Progress, so early media and early SDP negotiation can proceed predictably.
What is the difference between UPDATE and re-INVITE?
UPDATE can modify session parameters before the final answer or mid-dialog. re-INVITE is an INVITE inside an existing dialog, commonly used for mid-call media or session changes.
Which SIP requests are most important in VoLTE and VoNR?
The most important SIP requests are REGISTER, INVITE, ACK, PRACK, UPDATE, BYE, and CANCEL, with MESSAGE, OPTIONS, SUBSCRIBE, NOTIFY, REFER, INFO, and PUBLISH used in service-specific flows.