The modem defaults to VoLTE being disabled, because being enabled on networks that don’t support VoLTE could result in the phone not working at all from my understanding. Here are the steps to enable/disable that mode but please be very careful with this and double check everything you are typing.
To Enable VoLTE:
sudo screen /dev/ttyUSB3
ATE
AT+CGDCONT=2,"IPV4V6","ims"
AT$QCPDPIMSCFGE=2,1,0,0
ATE0
power cycle modem with kill switch
To Disable VoLTE:
sudo screen /dev/ttyUSB3
ATE
AT$QCPDPIMSCFGE=2,0,0,0
ATE0
power cycle modem with kill switch
I was able to get this working on Google Fi (which uses TMobile’s backbone in this case).