LTE VoLTE Mobile Terminated Call Procedure Call Flow
LTE VoLTE mobile terminated call is the end-to-end procedure used when an incoming IMS voice call reaches a UE over LTE.
This page follows the incoming branch from LTE reachability and paging into the SIP dialog and the later bearer path used for the active voice session.
Introduction
If the UE is idle, the call usually begins with LTE reachability restoration such as Paging and Service Request. Once the UE is reachable, the incoming SIP INVITE arrives and the normal provisional and final session setup continues.
The main nodes are the UE, eNB, MME / EPC, and the IMS path.
What Is VoLTE Mobile Terminated Call Procedure in Simple Terms?
- What starts the procedure: An incoming IMS call targets a UE that is attached to LTE.
- What the UE and network want to achieve: Restore LTE reachability if needed and complete the SIP dialog for the incoming call.
- What success looks like: The UE is paged if needed, receives INVITE, and the call reaches final SIP establishment.
- What failure means: The incoming call fails before the UE becomes reachable or before the SIP dialog is completed.
Why this procedure matters
Mobile terminated VoLTE issues often sit between LTE reachability and IMS call control, so this page is useful when the failure point is not obviously radio-side or IMS-side.
Quick Fact Sheet
| Procedure name | LTE VoLTE Mobile Terminated Call Procedure |
|---|---|
| Domain | Incoming VoLTE call setup |
| Main trigger | IMS routes an incoming call toward the LTE UE |
| Start state | UE is LTE-attached and usually IMS-registered |
| End state | Incoming VoLTE call is established over LTE |
| Main nodes | UE, eNB, MME / EPC, IMS |
| Main protocols | SIP, NAS, LTE access support |
| Main success outcome | Incoming call reaches the UE and enters active voice state |
| Main failure outcome | Call fails in reachability restoration or SIP establishment |
| Most important messages | Paging, Service Request, INVITE, 180 Ringing, 200 OK |
| Main specs | TS 24.229, TS 23.228, TS 24.301 |
Preconditions
- The UE is attached to LTE and has an IMS registration or reachability context.
- IMS can route the incoming voice session toward the UE.
- Paging and service restoration can succeed if the UE is idle.
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
IMS / EPC eNB / MME UE
|--Paging / SIP route-------------------->|
|<--Service Request continuation----------|
|--INVITE-------------------------------->|
|<--180 Ringing---------------------------|
|<--200 OK-------------------------------|
|--ACK---------------------------------->|Major Phases
| Phase | What happens |
|---|---|
| 1. Reachability recovery | LTE paging or service restoration makes the UE reachable if it was idle. |
| 2. Incoming SIP delivery | IMS delivers INVITE toward the UE. |
| 3. Call alerting and progress | The UE returns ringing or progress indications. |
| 4. Final answer | 200 OK and ACK complete the incoming call setup. |
Step-by-Step Breakdown
Step 1: Restore UE reachability if needed
Sender -> receiver: EPC / eNB -> UE
Message(s): Paging and Service Request
Purpose: Bring the UE back into active LTE signaling if it is not already reachable.
State or context change: The UE can now receive the incoming SIP dialog cleanly.
Note: Many mobile-terminated call failures start here rather than in SIP itself.
Step 2: Deliver the incoming INVITE
Sender -> receiver: IMS -> UE
Message(s): INVITE
Purpose: Present the incoming voice session to the UE.
State or context change: The UE now has the incoming dialog context and can alert the user.
Note: If the UE never sees INVITE, check reachability restoration before assuming an IMS routing problem.
Step 3: Return progress
Sender -> receiver: UE -> IMS
Message(s): 180 Ringing or 183 Session Progress
Purpose: Show that the incoming call is progressing on the UE side.
State or context change: IMS knows the call has reached the UE and is moving toward answer.
Note: 180 and 183 explain different parts of the user-visible call behavior.
Step 4: Answer the call
Sender -> receiver: UE -> IMS and IMS -> UE
Purpose: Complete the incoming call dialog.
State or context change: The call enters established voice state and the bearer branch can continue as needed.
Note: Use the answer point as the pivot between terminating-side alerting and active voice analysis.
Important Messages
| Message | Protocol | Direction | Purpose in this procedure | What to inspect briefly |
|---|---|---|---|---|
| Service Request | NAS | UE -> MME | Restores LTE service after paging when the UE was idle. | Check whether service restoration completed before SIP delivery. |
| INVITE | SIP | IMS -> UE | Delivers the incoming call to the UE. | Check whether the UE actually received the incoming dialog. |
| 180 Ringing | SIP | UE -> IMS | Shows that the UE is alerting for the incoming call. | Check whether alerting happened and how long it lasted. |
| 200 OK | SIP | UE -> IMS | Confirms that the incoming call was answered. | Check the answer timing and whether the dialog advances to ACK. |
| ACK | SIP | IMS -> UE | Completes the successful INVITE transaction for the MT call. | Check whether final confirmation reached the UE after answer. |
Important Parameters to Inspect
| Parameter | What it is | Where it appears | Why it matters | Common issues |
|---|---|---|---|---|
| Reachability timing | The interval between paging and SIP delivery. | Paging and service restoration branch | Shows whether the UE became reachable in time for the call. | The UE is paged, but INVITE is delayed or lost afterward. |
| Called identity | The identity used to route the incoming call. | INVITE | Explains whether IMS targeted the correct UE service profile. | The call reaches IMS but targets the wrong service identity. |
| Alerting state | Whether the UE returned ringing or progress. | 180 / 183 | Shows whether the incoming call reached the UE user side. | The call rings in IMS logic but not on the UE side. |
| Answer timing | The point where the UE answers the call. | 200 OK | Useful for correlating user action with SIP confirmation. | The call looks answered in one trace and not in another. |
| Bearer timing | When the voice bearer branch appears relative to answer. | LTE bearer branch | Shows whether the voice path was made ready at the right stage. | Answer succeeds, but the voice bearer branch is late or missing. |
Successful Completion
Success means the UE becomes reachable, receives the incoming INVITE, and the SIP dialog reaches final establishment for the voice session.
Common Failures and Troubleshooting
| Symptom | Likely cause | Where to inspect | Relevant message(s) | Relevant interface(s) | Likely next step |
|---|---|---|---|---|---|
| Incoming call fails before SIP reaches the UE | Paging or service restoration did not make the UE reachable in time. | Paging, service request, and the gap before INVITE. | Service Request, INVITE | LTE access, Gm | Resolve LTE reachability first. |
| INVITE reaches the UE but the call never answers cleanly | User-side alerting, SIP progression, or service logic breaks before final answer. | INVITE, 180, 183, and the absence of 200 OK. | INVITE, 180 Ringing, 183 Session Progress | Gm | Check whether the call stopped before or after user alerting. |
| Answer succeeds but the active voice path still fails | The bearer or media branch did not align with the successful SIP answer. | 200 OK, ACK, and later bearer handling. | 200 OK, ACK | Gm, LTE bearer path | Move into dedicated bearer and media-path analysis. |
What to Check in Logs and Traces
- Start by deciding whether the failure is in LTE reachability or in IMS call control.
- Use INVITE reception as the pivot between the LTE side and the IMS side.
- Correlate the answer point with the voice-bearer timeline afterward.
Related Pages
Related sub-procedures
Related message reference pages
Related troubleshooting pages
Notes
The mobile-terminated path is usually a combined reachability and SIP problem space. Do not skip the LTE paging and service-restoration branch when the incoming INVITE looks late or missing.
FAQ
Why is Paging relevant to a VoLTE MT call?
Because the UE may need to be brought back from idle reachability into active LTE signaling before the SIP dialog can be delivered.