For everyone who comes along on this. There is no VoLTE support on any Telstra based network.
This is not something that is going to be fixed, unless Broadmobi releases a firmware update that supports Telstra, which is very unlikely as the modem seems to be no longer updated. The only option is to switch networks or switch modems.
There was an investigation on using a new modem, which you can see here [Update: Mar 25th] Testing Quectel EM12-G WWAN 4G Modem on Librem 5 - #19 by alistair23. But call audio doesn’t work as it’s too distorted so that isn’t really helpful.
It seems like Vodafone and Optus work though, as long as you have the latest firmware.