AT&T’s implementation of Wi-Fi Calling requires an iPhone 6, iPhone 6 Plus, iPhone 6s, or iPhone 6s Plus running iOS 9 as well as a postpaid wireless plan. If you meet those requirements, you can enable Wi-Fi Calling by going into Settings on your iPhone, then selecting Phone, and then enabling Wi-Fi Calling.
Wi-Fi Calling will automatically routes calls and texts over a Wi-Fi signal if you’ve got poor or no cellular reception. If you’re using Wi-Fi Calling, you’ll see “AT&T Wi-Fi” in the status bar of your phone. It’s worth noting that making calls over Wi-Fi won’t count against your voice plan usage, but sending text messages using Wi-Fi Calling will count against your usage limits.