AT&T has been promising WiFi calling for some time, and earlier this week the FCC finally gave the carrier approval. WiFi calling is now available select devices on AT&T, which means you now have another way to make calls in addition to the regular cellular network. This new feature is a good thing for their coverage, it compliments their current coverage and gives you less of a chance of dropping a call.
Once you turn the service on, your device will atomically use WiFi to make calls when you have no or little cell signal. Currently, the only devices that support WiFi calling are the iPhone 6s, iPhone 6s Plus, iPhone 6, and iPhone 6 Plus. They also must be running iOS 9, which enables WiFi calling. For the most part this new technology or rather feature will be working in the background, you keep your same phone number and the experience of talking on the phone should be the same.
Keep in mind that AT&T is not the first to enable WiFi calling, as T-Mobile launched it earlier this year. With that being said AT&T does have a larger network and was the first carrier to get the iPhone. It will be interesting to see if Verizon Wireless or Sprint makes the jump to turn on WiFi calling.