This WiFi calling is entirely a new technology, previously it was only in iOS 8 that Apple finally introduce the feature to other devices. At the moment only T-Mobile and Sprint customers are able to use the feature, although AT&T has promised to enable the service as well, and interestingly enough it seems that they have with new and latest iOS 9.
As per some sources this new feature WiFi calling will be enabled for some iOS 9 users with AT&T. Unfortunately there is no word on when AT&T will launch the service for iOS users, but the fact that it was found enabled in iOS 9 is pretty encouraging. iOS 9 is currently in beta and has been scheduled for a fall release, possibly alongside the release of the new iPhones. It has been speculated that is when AT&T’s iOS WiFi calling feature will be activated as well.
However their is no official word from AT&T, One of the readers who is an AT&T employee has stated that his store has yet to receive any instructions with regards to the feature.