I would recommend reaching out to your current service provider for further assistance regarding Wi-Fi Calling. Wi-Fi Calling is a carrier-specific feature.
I've been wondering the same thing. the phone i have is a Verizon unlocked but I use with at&t. by placing sim into cell. this allows me to at least have service. I can not access the wifi calling options either. the device is capable. you may have to do the same I need to do which is actually activate the phone with your new carrier and also request the unconditional call forwarding be removed.
good luck though there are very few employees,including supervisors, in the technical support dept of these major cellular service companies that have any in depth knowledge.
yes unlocked my att purchased phone. switched to Verizon. assoon as it s which it was no longer in any of my menus even the ones I tried talking with Verizon text