The PhoneScoop info page for the Galaxy Note 3 (GSM) shows that the T-mobile version and the AT&T version support all of the same modes (technologies + frequency bands), EXCEPT that that the AT&T version does NOT support WCDMA 1700. According to that same page, HD Voice (higher-quality, wideband telephone audio) is supported by the T-mobile version ONLY. I read elsewhere that AT&T is required to provide unlocking codes for phones sold by them (except for iPhone and phones sold exclusively by them) on request, pursuant to a class-action consent decree. With T-mobile, there are apparently waiting periods that can range from 40 days of being a T-mobile customer to 90 days after exercising a "subsidy unlock," whatever that is. Of course, you can always try to unlock your phone other than through the carrier. There may be more differences between the T-mobile and AT&T versions, but those were the major points I took away from what I read on the subject.