Juli Clover, writing for Mac Rumors:
AT&T has flipped the switch on Wi-Fi calling, making it available to customers with eligible plans that are running iOS 9. MacRumors has received tips from customers who were able to activate Wi-Fi calling and we were able to activate the feature on our own iPhones. A number of readers in our forums are also having success activating Wi-Fi calling.
Wi-Fi calling is a feature that lets calls be placed over a wireless connection when cellular connectivity is poor, functioning much like an AT&T M-Cell does now. It’s similar to Apple’s own FaceTime Audio feature, which also routes calls over a Wi-Fi connection.
If AT&T is your carrier, you find yourself with a poor cell signal but with solid access to WiFi, WiFi calling is for you. It won’t eat up your data plan and your signal will go from nonexistent to excellent.