For some time now T-Mobile and Sprint users have had the option to make calls over Wi-Fi. But beginning with iOS 9 AT&T users have this ability too. Essentially, this uses your Wi-Fi internet connection to connect into your provider’s system rather than connecting via the cellular tower, allowing you to make calls and send SMS where you have good WiFi but a lousy cell signal. If on a metered voice plan, calls will still come off your minutes as normal since you are still making a phone call through their system. It’s just the first leg of the connection that you’re bypassing.
To enable, you will need an iPhone 6 or later, and turn the option on in Settings > Phone > Wi-Fi Calling.
Now when WiFi Calling is active, your carrier name will switch from “AT&T” to “AT&T Wi-Fi”.
If you run into trouble see Apple’s help document or AT&T’s information on the subject.