Remember how AT&T begrudgingly promised to add Wi-Fi call support for the iPhone after Apple introduced the feature with iOS 8? It only took until the third iOS 9 public beta for it to show up, so now we only have to wait for the public release, and for AT&T to enable the feature on its cell network.
AT&T Wi-Fi calling support finally shows up in iOS 9 beta
Wi-Fi calling is a feature that lets smartphone users send and receive calls over wireless networks instead of relying only on cell towers. The benefit is that you aren't limited to potentially weak cell signals, and calls that happen on Wi-Fi networks don't count against your monthly voice call caps.
T-Mobile jumped on Wi-Fi calling as soon as Apple added the feature to the iPhone, and so far has been the only carrier supporting the feature. AT&T eventually said it would add Wi-Fi calling support, but couldn't understand why consumers would want it considering how awesome their coverage areas are. On a side note, AT&T clearly hasn't tried using their own cellular network in Boulder, Colorado.
Adding a Wi-Fi call setting option for AT&T in iOS 9 doesn't seem like the sort of thing Apple would bother with if the carrier wasn't planning on enabling the feature soon. Since it's there, maybe AT&T really will follow through and give its iPhone customers Wi-Fi calling support some time this year.