I apologize for the inconvenience caused to you.
I would like to inform you that OnePlus hardware supports VoLTE, but software doesn't support VoLTE. Already our development team is working on it, they will try to fix this issue in upcoming OTA updates, I request you to please wait for upcoming OTA updates. I really appreciate your time and patience.
Thank you for contacting OnePlus customer support. Have a great day.