VOLTE: How the VoLTE call establishes for LTE network and legacy network ?

Question

There are two scenarios of VoLTE call.
1. Originator and Receiver both are connected in LTE network.
2. Either one of them is not in the LTE coverage .

How the signalling messages flow between originator and receiver for both are scenarios ?

Answer 1

I would Say 3 scenarios:
1. LTE (VoPS/VoLTE capable) to LTE (VoPS/VoLTE capable)
2. LTE VoLTE to Non VoLTE eNB but LTE, this will be like Scenario 2 in your case.
3. VoLTE to PSTN

1. VoLTE to VoLTE call
   Both UE's are registered on IMS core N/W
   One of them initiates VoLTE call via SIP invite.. check call flow online ......

2. VoLTE to Non VoPS supported.
   VoLTE capable UE (MO)will initiate SIP Invite to terminating UE (MT), IMS server try to convert the Terminating user telephone number to SIP URi and conversion will fail as the MT user not registered on IMS server which is required for routing, which indicates IMS server need to reach out to BGCF (break out gate way control function) for further routing of the call in Legacy CS N/W.... and call will progress where MO will be still having SIP messages and MT will be legacy CS call flow.
   MT will get paging request on LTE and MT UE needs to do CSFB and go to legacy CS N/W.

3 VoLTE to 3G/ PSTN call

MO assuming VoLTE capable and MT( assuming PSTN/ phone or 3G )
Same as scenario 2 where MT doesn't have to do CSFB as it is already on CS N/W.

Hope this helps