Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Yes all iPhones on iOS 8 that work on T-Mobile can do wifi calling.
It's not a device feature but a carrier file that enables it when a T-Mobile sim is in.

Perfect. My Note 4 has Wi-Fi Calling issues. My 5C should come in a few days and will be my back up, especially when I go to the gym.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.