This could be due to AT&T testing it before the official launch, so if you don’t have this feature, it’s probably due to that cause.
Mac Rumors has reported that “WiFi calling was first introduced with iOS 8, and shortly after the feature debuted, AT&T announced plans to support it. The company has not made any further announcements, but it is likely an official launch will come after iOS 9 is released to the public in September. An AT&T employee who is also a MacRumors reader says that his store has not received word on an upcoming launch, so official word on further availability of WiFi calling may still be a few weeks off. “
“WiFi calling is a feature that lets calls be placed over a wireless connection when cellular connectivity is poor. It is similar in function to Apple’s own FaceTime audio feature, and has thus far been limited to T-Mobile and Sprint users.“