I wondered why VOLTE was not working here in Germany with Vodafone carrier. So I tried older solutions.
Here is what worked for me:
Install new Oneplus Logkit:
Android 9 version also works on Android 10.
Found the helping post at https://forum.xda-developers.com/sho...9&postcount=23
Then do the following
Open the dialer: *#800# --> oneplus Logkit --> Function Switch --> enable VoLTE + VoWifi switch
found at https://forum.xda-developers.com/one...riers-t3817542
After reboot VOLTE was instantly on and working.
Aa always: Do at your own risk. I am not repsonsible for bricked HW, nuclear war and alien invasions