This blockage is due to the mobile network operators.
If OnePlus does not want to pay horrendous sums to Swisscom, the mobile phone will not be sold there and the corresponding profiles will not be approved.

OnePlus mobile phones would support both, and there are also suitable profiles from Denmark via some detours.

I have noticed the same thing with my phone. Technically, the OP 7 pro is able to make VoLTE (and VoWifi) calls, but as I understand it, the provider (Swisscom in my case) has to activate this service first. We can only hope that this will happen soon.