The iPhone model I'm talking about is the A1633, which is different from the T-Mobile sold one I think.
Will band 12 extended range coverage work on the phone, with data, voice and texting?
Will WiFi Calling and HD voice work?
Also, will I experience any other differences between a T-Mobile sold iPhone 6S compared to an AT&T one?
Yes, it will work. Apple put wifi calling into their software and Band 12 in it. That model was the most widely compatible model. I don't see any reason that you cannot use this phone on T-Mobile from reading about the compatibility and other features.
It will work fine just make sure to get it unlocked if you bought it from AT&T. If you got it straight from Apple it will already be unlocked and alls you will need to do is buy a plan/activate a sim with T-mobile and plop it in.
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