The current supported list for 4G Calling is limited to quite a few handsets.
The OP 6 has a modem capable of the VOLTE capability but it is not provisioned by the network.
I would be very happy if you support this I cannot see any negatives to this.
Best answer by The_Calluminati
I've got the 1+6t and I think I've found a work around for the wifi calling option.
Dial *#800# > oneplus Logkit > Function Switch > Then tick VoLTE switch and VoWifi Switch > Reboot.
This should bring the wifi calling options online under the SIM settings. Hope this helps!