AT&T Wi-Fi Calling
According to MacRumors, iOS 9 beta 5 added Wi-Fi calling features for those who own a device on AT&T's network.
As of right now, the feature doesn't work. iOS 9 beta users trying to flip the option on are greeted with a message that says that the feature is currently not available. That said, the inclusion of the feature probably means that AT&T will flip the feature on at some point in the near future, maybe next month alongside the iOS 9 release.
Wi-Fi calling is currently available on carriers like T-Mobile and Sprint and it's a feature that AT&T subscribers have been requesting for a long time.