It's definitely a welcome upgrade, but note that the feature only works with the iPhone 6, 6 Plus, 6s, 6s Plus and, when it's released, SE running Apple's latest mobile platform. Besides updating its WiFi calling service, AT&T now allows iPads and Macs connected to your iPhones via Numbersync to make calls on a cellular connection.

[Thanks, Vladimir Pimentel]

Update: The recipient does not need to have WiFi calling for the feature to work. It works automatically when you're abroad and can call any US phone number without extra charges.