Are there other US carriers that offer WIFI calling besides T-Mobile? Does AT&T?

I like T-Mobile. And I really like their WIFI calling. Like when I go out of the country all I need is WIFI and I can make calls just like if I were in the USA. But I really wanted the Nokia Lumia 1520 that AT&T had. Well they don't offer it anymore, but when the next Windows flagship phone comes out I'd like to get it. I just doubt T-Mobile will ever sell such a phone.

PS, currently we (that is, my wife) have a Nokia Lumia 925 with T-Mobile.

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