"I’ve been testing it — at home, and at public Wi-Fi hotspots in coffee shops, airports and hotels in Maryland and Massachusetts. In my tests, it worked just as advertised. It simply required me to go into the iPhone’s settings and toggle a switch called 'Allow Wi-Fi Calls' on the T-Mobile version of the new phone."
AT&T, too, is a carrier with strong nationwide coverage. Like Verizon, the company has made Wi-Fi calling a lower priority – it feels it’s not really needed, especially given AT&T’s current lack of support for VoLTE. According to Ralph de la Vega, the CEO of the company’s mobility and enterprise unit, Wi-Fi calling is coming to AT&T sometime in 2015.
Poor Sprint. They can't seem to do ANYthing right. If ever a carrier needed wifi calling its Sprint, and right now. I came to Verizon from Sprint because of their terrible network. 3G only for data in my area and it's literally dial-up modem speed. And driving just 15 minutes north I lose ALL network connectivity; no voice or data. Phones make good paperweitpghts though!