I have a few updates on getting this working.
According to 4G VoLTE calls confirmed working with carrier x the PinePhone works with Telstra and VoLTE. I did a test today and my PinePhone seemed to stay connected to the LTE network during the call (although audio didn’t work).
The post Quectel EM12-G WWAN 4G Modem on Librem 5 also did some testing on using a Quectel EM12-G in the Librem 5.
The Quectel EM12-G is similar to what is in the PinePhone (the actual PinePhone EG25 modem looks to be too large for the Librem 5). The Quectel EM12-G datasheet indicates that it is compatible with Telstra as well
So I’m hoping that something like this could be swapped out in the Librem 5 and then used to get VoLTE.