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

Stephen RoblesAT&T, iPhone