It’s available for iPhone 6, 6+ and 6S and 6S+ AT&T customers with iOS 9. You’ll have to go to your phone’s settings (Settings > Phone) and flip it on manually:
You’ll then have to enter in an emergency address in case you’re ever in need of assistance. Wi-Fi isn’t as traceable as your cellular connection, so if you were to call 911, AT&T needs the extra information to identify you.
The feature is quite handy when you have a Wi-Fi connection but your cell connection sucks. The phone will switch over to Wi-Fi automatically.