Basically, if you have poor cell signal in your house or business but you're connected to Wi-Fi, your iPhone will use the Internet connection to place phone calls and receive texts. iPhone 6 and above with iOS 9 can turn it on in Settings > Phone > 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. AT&T customers can turn on Wi-Fi calling by going to the Phone section of the Settings app and toggling on the Wi-Fi calling feature. From there, there are a set of steps to walk through, including entering an emergency 911 address. Wi-Fi calling is available on the iPhone 6, 6s, 6 Plus, and 6s Plus running iOS 9.