Big question but simplified summary is below.
If the unit in question has the ability to connect to the networks IMS (IP Multimedia Subsystem) then VoLTE will work with the unit i.e. the voice call will be routed over LTE. To connect to the IMS there need to be a number of things in place.
- SIM subscription (obviously).
- IMS being capable.
- Firmware on the unit being capable and configured correctly. This is the big one as in an effort to include everyone the VoLTE spec was very loosley specified unlike in 2/3G where it is an integral part of te spec, this means implementations vary from OEM to OEM and operator to operator. As a result having a generic firmware which will work on any given operator is not possible, you need to go to each operator and perform an integration exercise. For the handsets where the operator buys them it is not a problem i.e. we will order 100K iPhones if you come and perform the integration exercise, provide a custom firmware, etc.
The above may or may not change with the operators coming to common concensus but given 99% of the voice traffic goes over their own handsets it is not a big deal for them. Additionally 99% of the IoT application/traffic is data so this is not a great driver for them.
If the firmware is not capable of connecting to the IMS then the unit will be redirected back to 2G and the voice call will take place on that. This will also put any data session on hold as no one supports simultaneous voice and data sessions on 2G.
With regards Sierra VoLTE capable units due to the above restrictions and limited pull from the customer base (as 99% of our customers are data only with the exception of a few specific applications where it is critical) it is limited to the North American carriers at the moment.