I bought the T-MOBILE model because it was the only non-contract unlocked version available when the 6S+ was released. I have done this in the past with older versions too without any issues. For some reason, when phones are released, Apple doesn't offer a straight unlocked version with no carrier associated. They add it later and also include the little pin to eject the SIM tray, and don't include a tmobile sim.
But according to Apple, it is the exact same phone with the exact same features.
I use AT&T. I tried the wifi calling a few days ago, and the option was there, but it kept giving me an error when I tried to activate it.
However, today, I tried it, and it worked. So obviously, this was on the AT&T end, and not the Apple end.