AT&T Wi-Fi Calling Only on iPhone 6 or Newer
Tried activating Wi-Fi calling on my wife's iPhone 5s, no dice. Here's why:
"We’re proud to offer Wi-Fi Calling on iPhone, the phone that’s absolutely beautiful and simple to use. Enjoy Wi-Fi Calling on these models**: • iPhone 6s • iPhone 6s Plus • iPhone 6 • iPhone 6 Plus"
Read here: Wi-Fi Calling from AT&T