I suspect it will coming to us at some point. As it one of the features listed in their questionaire they released a few months back.
I am the end of my sim only contract and the lack of Wifi Calling is starting to become a bugbear.
Fed up with having to use Smart Talk which charges me to call mobiles, when I have loads of free minutes available on my mobile plan which I just can't use because the BT signal I get is so poor. There is no way of knowing what network a mobile number you are calling is on. It goes against the grain, and I'm ready to jump ship.