It's a software feature that's integrated into the operating system. So far, T-Mobile has not been able to offer wifi calling on the iPhone because of the way Apple creates and distributes it's iOS. You can try something like Google Voice to get similar functionality, but T-Mobile will most likely tell you there is no ETA for integrated wifi calling on any iPhone.
Thanks UncleSpoon. You are absolutely correct, turns out that WiFi calling is only available on T-Mobile "branded" phones. If you bring an unlocked phone like the Google Nexus or iPhone to T-Mobile, you will not be able to get WiFi calling.
Maybe T-Mobile can develop an app for iPhone that will give it WiFi calling capability? We will see. I realize that there are VOIP options such as Skype, Google Voice, Viber and Orange, but T-Mobile's WiFi calling is unique and well worth it for our particular situation. WiFi calling would allow us to use the phone in our family's home in Brasil, just as if we were at our home in the USA. Incredible that it is a free service!