From my understanding of the issue - the answer is Yes
If you take a look here -
Apple - iPhone 6 - View countries with supported LTE networks.
The T-Mobile phone is physically the same as the ones sold by Verizon and AT&T
I had a chat with Apple support and they said that the Wi Fi calling is a carrier feature
In other words as long as you have an active T-Mobile account - the WiFi calling should work on variants 5S and above
Please let us know if you have any questions