I have the international model from B&H Photo Video (great company, BTW, I have been a satisfied customer for over 50 years). I have used it on both T-Mobile and AT&T and it works fine with both. I bought it because i travel abroad a lot and it covers more LTE bands than any other phone I have found; (Bands:1, 2, 3, 4, 5, 7, 8, 12, 17, 20, 28, 38, 40). 2, 4, 12 and 17 will cover most of the GSM networks in the US. (Note that not many phones support 12 which is a new low frequency band introduced by T-Mobile and they are using it a lot.)
I am very happy with my 640XL and I think you will find that it will work on any GSM carrier that you want in the US. (Don't take my word for it, go to the B&H site and work your way through it. Takes a few minutes, but you can check it out to your own satisfaction.)
Best,
<TED>