AT&T announced today that it's bringing Wi-Fi calling to select Android devices. As you'd probably expect, they need to be AT&T devices, as opposed to unlocked devices that don't have the carrier's software installed.
The LG G4 is going to be the first device to receive the service since then. In fact, that's the only device that AT&T specifically called out as getting the service, saying, "We expect Wi-Fi calling to be available on more Android devices soon."
Of course, even though you're not exactly connecting to cellular data, normal billing applies.
As one might expect, there is no news for Windows phone users. Coming from the US carrier that's known for carrying the devices, it's sad news, especially when it comes alongside falling market share numbers.
AT&T notes that in order to use Wi-Fi calling, you'll need the following:
A compatible device
A postpaid wireless account set-up for HD Voice
A Wi-Fi internet connection