My BIL is on AT&T also, so I guess not. We have AT&T, but we have 6s's so no problem. That seems illogical, but perhaps someone can explain it to me?
Saintsfan doesn't have it on HER 5S
I think AT&T decided to only support WiFi-Calling on the iPhone 6 and above because the 6 and up support VoLTE. Because of that, they can do handoffs between the cellular network and WiFi seamlessly. Otherwise, your call would drop when you moved out of WiFi range or the quality of the network dropped.