Finally!  A feature you probably didn’t know you wanted.  If you want this you may have to call AT&T to get it activated on your phone.  Some accounts have HD Voice turned off.  Mine did.  You might have to argue with your AT&T tech a bit about the service being available as it just came out today 10-08-2015 according to my AT&T service tech.    Read more on TechCrunch.com below.

After being promised Wi-Fi calling for over a year by Apple and AT&T…it’s finally here. It’s available for iPhone 6, 6+ and 6S and 6S+ AT&T customers with iOS 9. You’ll have to go…

Source: Wi-Fi Calling Finally Comes To The iPhone For AT&T Customers | TechCrunch