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LTE SMS over IMS in LTE Call Flow

call-flow LTE | IMS | SMS | SIP

LTE SMS over IMS is the messaging procedure used when SMS service is carried through the IMS path instead of a legacy SMS path.

This page focuses on the SIP transaction used for IMS-based SMS delivery over LTE and how it relates to IMS registration and service availability.

Introduction

SMS over IMS usually depends on successful IMS registration first. The actual message transaction is then carried through SIP, typically using MESSAGE and the related success or failure responses.

The main nodes are the UE and the IMS path, with LTE access and packet service supporting that SIP transaction underneath.

What Is SMS over IMS in LTE in Simple Terms?

What starts the procedure: The UE or network needs to send an SMS while IMS service is active over LTE.
What the UE and network want to achieve: Deliver the SMS through the IMS path instead of a legacy SMS route.
What success looks like: The MESSAGE transaction is accepted and the IMS SMS service completes cleanly.
What failure means: IMS SMS service is unavailable, rejected, or incorrectly routed.

Why this procedure matters

SMS over IMS can fail even when LTE data works, so this page helps separate IMS messaging issues from generic LTE access problems.

Quick Fact Sheet

Procedure name	LTE SMS over IMS in LTE
Domain	IMS messaging over LTE
Main trigger	An SMS is sent or received through IMS service
Start state	IMS registration and LTE packet service are already active
End state	IMS SMS transaction completes successfully
Main nodes	UE, IMS
Main protocols	SIP, IMS SMS signaling
Main success outcome	SMS is delivered through the IMS path
Main failure outcome	SMS over IMS fails or falls back incorrectly
Most important messages	MESSAGE, 200 OK
Main specs	TS 24.341, TS 24.229

LTE SMS over IMS in LTE — Click the diagram to open the full-size in a new tab.

Preconditions

The UE is IMS-registered over LTE.
SMS over IMS service is provisioned in the network.
The IMS packet path is stable at the time of the message transaction.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Starts or receives the LTE voice or messaging service and exchanges SIP signaling with IMS.
eNB	Carries the LTE radio side used for IMS-capable packet service.
MME / EPC	Preserve LTE access, bearer, and paging continuity behind the IMS transaction.
P-CSCF / S-CSCF	Handle the SIP signaling path used for registration, call control, and service continuity.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the radio access needed before and during IMS service use.
S1-MME	eNB <-> MME	Carries LTE control-plane continuation behind paging, service request, or bearer handling.
Gm	UE <-> P-CSCF	Carries SIP requests and responses between the UE and IMS.

End-to-End Call Flow

UE                 IMS
|--MESSAGE----------------------->|
|<--200 OK------------------------|
|   IMS SMS transaction complete  |

Major Phases

Phase	What happens
1. IMS SMS readiness	IMS registration and SMS capability are already active.
2. MESSAGE transaction	The SMS payload is sent through the SIP messaging path.
3. Final response	IMS confirms success or returns a failure response.
4. Service completion	The IMS SMS procedure ends or moves to failure handling.

Step-by-Step Breakdown

Step 1: Start the IMS SMS transaction

Sender -> receiver: UE or IMS -> other side

Message(s): MESSAGE

Purpose: Carry the SMS transaction through the IMS path.

State or context change: IMS now handles the SMS exchange as a messaging service rather than a voice dialog.

Note: Use MESSAGE as the main pivot when SMS service works inconsistently over LTE.

Step 2: Confirm or reject the SMS transaction

Sender -> receiver: IMS -> UE or peer

Message(s): 200 OK or failure response

Purpose: Show whether the IMS SMS transaction was accepted.

State or context change: The SMS over IMS attempt is now known to be successful or failed.

Note: A clean MESSAGE / 200 OK pair is the basic success pattern for IMS SMS delivery.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
MESSAGE	SIP	UE or IMS -> other side	Carries the IMS SMS transaction.	Check destination identity, transaction timing, and whether the IMS SMS path was actually used.
200 OK	SIP	Other side -> requester	Confirms successful IMS SMS transaction handling.	Check whether the success response follows the correct MESSAGE transaction.
404 Not Found	SIP	IMS -> requester	Useful failure response when the destination or service route cannot be found.	Check whether the destination identity or IMS routing is wrong.
480 Temporarily Unavailable	SIP	IMS -> requester	Useful failure response when the IMS destination or service is temporarily unreachable.	Check whether the destination service is reachable at transaction time.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Destination identity	The SMS target identity.	MESSAGE	Explains where the message is being routed.	Wrong target causes routing failure.
IMS registration state	Whether the UE was correctly IMS-registered before the SMS attempt.	Before MESSAGE	SMS over IMS often fails when the registration state is incomplete.	LTE data works, but IMS registration was not stable.
Transaction result	The final SIP response to the message transaction.	200 OK or failure response	Shows whether the transaction was accepted, not found, or temporarily unavailable.	The MESSAGE is sent, but the real result is not checked.
Service routing	The IMS path used for SMS service.	MESSAGE and response path	Useful when SMS is misrouted or unexpectedly falls back.	The message uses the wrong service path.
Timing and retransmission behavior	The transaction timing around MESSAGE and its response.	SIP ladder	Important when SMS delivery feels delayed or inconsistent.	Repeated MESSAGE attempts are mistaken for multiple user actions.

Successful Completion

Success means the IMS MESSAGE transaction completes cleanly and the SMS is delivered through the IMS service path.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
MESSAGE gets no clean success response	IMS messaging path, registration state, or service routing is broken.	MESSAGE and the final SIP response.	MESSAGE, 200 OK, 404 Not Found, 480 Temporarily Unavailable	Gm	Check IMS registration and routing before assuming general LTE failure.
SMS falls back unexpectedly	IMS SMS service is not available even though LTE packet service works.	Registration state and the transaction result.	MESSAGE and returned failure response	Gm	Decide whether the problem is service provisioning or IMS reachability.
Repeated SMS attempts appear in traces	Retransmission or retry behavior is being confused with separate user actions.	Transaction timing and repeated MESSAGE use.	MESSAGE	Gm	Correlate transaction timing before counting separate attempts.

What to Check in Logs and Traces

Confirm IMS registration before troubleshooting the SMS transaction itself.
Use the MESSAGE / response pair as the main SMS over IMS correlation point.
Separate IMS SMS routing failures from LTE packet-connectivity failures.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

SMS over IMS uses the IMS service path rather than a classic voice dialog, so it should be analyzed as an IMS messaging transaction, not as a reduced VoLTE call.

FAQ

What SIP method is used for SMS over IMS?

MESSAGE is the main SIP method used for the IMS SMS transaction.

Can SMS over IMS fail while LTE data still works?

Yes. LTE packet service may be healthy while IMS registration, routing, or IMS messaging service is still broken.