Verizon iPhone 6 on T-Mobile - wifi calling?

[jntdroid]

We're looking at possibly trying out T-Mobile and both have Verizon iPhone 6's. I know we won't have an issue popping a Tmo sim into our phones as they're the same model and Verizon's are sim unlocked out of the box. My question is this - will we be able to do wifi calling? Is that a T-Mobile iPhone only thing? It looks like it's one little setting in iOS 8, but is that setting enabled when a Tmobile sim pops in, b/c I'm not seeing it now (obviously Verizon doesn't support it). The Tmobile guy told me it wouldn't work, but he also told me they're different models - which isn't true. Now, if the IMEI has to be a registered Tmobile IMEI - that I could understand. But other than that, I'm not sure what would keep us from doing wifi calling on Tmobile with our Verizon 6's...

TIA!