I’m not sure this is something that is possible…
Reading things like this https://forums.whirlpool.net.au/archive/9m01z7np it seems that we need to install a MBN file on the modem.
Page 53 of this Quectel documentation includes a AT command to 4.4.1. AT+QMBNCFG="List" Query Imported MBN File List. They even mention Telstra 
That command doesn’t work on my BM818. So I’m assuming that not all Quectel modems support it.
According to this post there is a full list of AT commands available. There is no mention of MBN, Telstra or VoLTE there.
EDIT: Not all of the commands listed in https://www.waveshare.net/w/upload/6/68/SIM7500_SIM7600_Series_AT_Command_Manual_V2.00.pdf pointed to from Using AT commands and commandline to do stuff work either. So it seems that isn’t the correct list. Maybe someone from Purism has a list of AT commands?
So unless somone can figure out a way to add extra MBNs, update the firmware or replace the modem we might all need to switch to Vodafone or switch phones