I've been using T-Mobile Wi-Fi calling and HD-Voice since they rolled out the service on one of my iPhones. Now, AT&T has added the same features almost a year later. But there's a hitch. You need to have a newer version of an AT&T SIM card and that requires a visit to your local AT&T store.
That's a point that the AT&T web site and media coverage fails to point out even though the web page lists many caveats, restrictions and limitations.
I'm excited that AT&T is adding Wi-Fi calling, at least until more buildings and hotels become DAS equipped. We're entering an era where land lines are becoming more and more extinct and LTE coverage in building gets patchy and spotty. Wi-Fi is more plentiful and far more available, thus getting coverage and network access in the mobile first era makes Wi-Fi calling essential, and removes the need for Skype or another VoIP service provider who already has Wi-Fi calling capability from being in place.
This opens up a whole other argument. Equal Access to the mobile operator's networks. But that's for another day.