I'm just wondering, because I am currently on vacay in the Philippines using my unlocked ATT Focus with a Globe SIM card and it picks up 3G signal, which is UMTS 2100. Isn't that the same band that Tmo USA uses? Why then, do I always read about people not able to use their unlocked Focus on Tmo 3G?
Just to add a bit to palandri's explanation, AWS is spread-spectrum and it transmits on one band and receives on the other. This is why unless the phone is specifically made for T-Mo it will not work on 3G on that network. Not only does it have to have the bands, but it also has to be able to transmit on one and receive on the other.
An unlocked AT&T phone in the US will work on T-Mo only on EDGE.