LTE VoLTE and IMS Procedures
Learn how voice works over LTE using IMS, including IMS registration, SIP-based call setup, in-call handling, emergency VoLTE, and SMS over IMS, and see how LTE bearer control and IMS session signaling work together.
Quick Links
IMS Registration
Call Setup and Handling
Emergency and Messaging
What This Covers
VoLTE is the LTE voice service built on top of the IMS architecture. The UE first needs LTE attach, IP connectivity, and IMS registration, then uses SIP signaling to create, modify, and release voice sessions while RTP carries the media stream.
The main nodes across these procedures are the UE, eNB, MME, SGW, PGW, and IMS functions such as P-CSCF, S-CSCF, I-CSCF, and HSS. Because of that split, VoLTE troubleshooting often crosses both LTE control-plane behavior and IMS session behavior.
These procedures directly affect call setup time, voice quality, emergency-call handling, and service continuity. If IMS registration fails, calling never starts. If paging or service request fails, mobile-terminated voice breaks. If dedicated bearer or SIP handling fails, the call may connect without stable media or drop unexpectedly.
IMS Registration Procedures
These procedures establish the UE presence in IMS and must complete before normal VoLTE calling can begin.
- LTE VoLTE Registration Procedure — Shows how the UE registers to IMS after LTE access and IP connectivity are available.
- LTE SIP Session Establishment over IMS — Helps map SIP dialog setup and IMS session control to the larger voice-service path.
VoLTE Call Setup Procedures
These procedures define how IMS establishes mobile-originated and mobile-terminated voice sessions over LTE.
- LTE VoLTE Mobile Originated Call Procedure — Shows how a UE starts a voice call with SIP INVITE and voice-bearer setup.
- LTE VoLTE Mobile Terminated Call Procedure — Shows how an incoming IMS call reaches the UE through paging, service request, and SIP setup.
Call Handling Procedures
These procedures manage dedicated bearer setup and SIP-driven call handling while the voice session is active.
- LTE Dedicated Bearer Setup for VoLTE — Shows how LTE bearer control provides the QoS path needed for voice media.
- LTE VoLTE Call Hold / Resume — Shows how SIP session modification is used to pause and resume media.
Call Completion and Continuity
These procedures release the call cleanly or preserve continuity when the voice session must leave pure LTE service.
- LTE VoLTE Call Release Procedure — Shows how SIP BYE and related LTE cleanup end the session.
- LTE SRVCC Procedure — Shows how voice continuity is preserved when the session moves toward legacy access.
Emergency and Messaging Services
These procedures cover emergency voice over LTE and SMS delivery through IMS instead of legacy fallback.
- LTE VoLTE Emergency Call Procedure — Shows how emergency voice uses IMS together with emergency-specific LTE handling.
- LTE SMS over IMS in LTE — Shows how SMS is delivered over IMS without falling back to 2G or 3G.