OnePlus does support VoLTE/VoWiFi, on a lot of their devices. So, having found some tutorials online, I now have it working on my OnePlus 8 Pro ;)
Rooting is not required to do this.
Doing this has possible risks - you can possibly destroy your EFS partition! If you break your phone, it’s your problem, not mine. https://www.getdroidtips.com/how-to-backup-or-restore-qcn-efs-on-qualcomm-devices/
This requires downloading the Qualcomm Product Support Tool (QPST), and the Qualcomm Unified Driver (QUD) package, to edit the configuration of the modem. Download the two and install them on a windows machine (just use the defaults).
After installing the APK’s, dial #*800* on your phone.
Tap “oneplus Logkit”, and navigate to the bottom.
There, tap “Function switch”, enable “VoLTE switch” (without rebooting), enable “VoWifi switch” and then reboot your phone.
Now, you can go into “Settings” → “Wi-Fi & network” → “SIM & network”, select your SIM card slot that houses
your Orange SIM, and activate “VoLTE” and “Wi-Fi Calling”
To configure the modem profile, dial #*801* and switch on “Full port switch”.
Plug in your phone and start “QPST” on your Windows PC.
This should show your phone as a COM port. In the example above, COM4.
Next, start “PDC”. On the drop-down at the top (next to Device) select your device. This should load the list of
Find Oversea-Commercial_DS in the list, right click it and pick “Deactivate” → “Sub0”, and “Sub1”.
Search for the Telefonica_UK_Commercial, right click it and pick “SetSelectedConfig” → “Sub0” and “Sub1”.
Finally, hit “Activate” at the bottom of the screen.
Reboot your phone, and you should have a VoLTE / VoWiFi icon in your phone’s icon bar ;)