Couldn't get wi-fi calling activated on my unlocked 6S+ and now I know why - it is only supported for the AT&T model numbers. So even though I use AT&T with the unlocked T-Mobile variant, and even though the ONLY difference between the two is that the T-Mobile model doesn't receive some new LTE band, wi-fi calling is a no-go. And it's too late to exchange mine for an AT&T version. Very disappointing