It doesn't look like T-Mobile will allow WiFi calling on these. Maybe they could but might think more people will buy a T-Mobile locked phone to get it than will go to another carrier if its not there. Of course their theory goes out the window (pardon the pun) when they don't offer a phone remotely close to these.
BUT I believe T-Mobile will loan a sort of mini-tower to people who have bad reception. It hooks to your router and provides a strong cell signal inside the building. The effect is the same: your calls go over the internet but the transfer happens outside the phone rather than in it. I don't know 100% if they don't do something dumb and block non T-Mobile phones, but if the reason for no WiFi calling for BYOD phones is technical and not misguided, those technical reasons wouldn't apply to BYOD phones and the mini-tower because the phone thinks its using a real tower.
The T-Mobile Cellspot (a femtocell) is a mini cell tower in your home, plumbed into your broadband. Your phone has full LTE coverage, and even has HD Voice.
It's limited to T-Mobile phones. Any phones; there's no need for registering any given phone for it to work.
But that covers only your phone. What about when you're out and about? I spent a bit of time in hospitals this year (as a visitor) and wifi calling saved me. There was no T-Mobile cellular penetration at all, so I simply attached to the facility wifi and turned on wifi calling.
No wifi calling means no phone for me. If I want a 950XL, I'll have to wait for T-Mobile to offer it or for T-Mo and MS to get together on how to make all the T-Mo functionality (including visual voicemail) appear on it.