BM818-tools (PureOS package to enable VoLTE):
sudo apt install BM818-tools
sudo apt update
(apply any package upgrades or dependencies)
Reboot phone.
EDIT: To test, while a call is in progress, open Terminal, type the command:
sudo socat - /dev/ttyUSB3,crnl
Enter your password.
Then this:
AT+BMRAT
It should return this, if VoLTE is in use:
+BMRAT: FDD LTE