AT&T has reportedly activated Wi-Fi Calling for iPhone customers with eligible plans who are running iOS 9. Wi-Fi calling is available on the iPhone 6, 6s, 6 Plus, and 6s Plus running iOS 9.
Wi-Fi Calling allows devices to place calls over a wireless connection when cellular connectivity is poor. When the feature is switched on, an iPhone will use the Wi-Fi network it is logged into for phone calls, improving the quality of the call.