Contact your carrier support and ask them if your device is supported by them
This is for all OP phones, however it discuses the 8 Series, but it should apply to any series as the protocols are the same.
[GUIDE] Enable VoLTE & VoWiFi on all devices NO ROOT.
Log Kit 1.0
There are newer versions but I would suggest trying this with the details provided on XDA.
It's work 👍... Thank you so much 🤗
You're welcome 👍
Will it work even if the carrier says they do not support volte on oneplus?
If they support it on the network it might. Try it and let us know.
I have already VoLte support Before 11 update.. after 11 update VoLte options was missing.
Use this link may help
[Community Workshop X Shot On OnePlus August] – Nightscape
by Daniel D., 2021-08-01