Can a ATT 3g phone work on T-mobiles 3g if one frequency matches?

[ssr]

HP Ipaq Glisten is a world phone, it uses these Frequencies
GSM 850 / 900 / 1800 / 1900 / UMTS(3g) 850 / 900 / 2100

I read that T-mobiles 3g Frequencies are 1700/2100 MHz

Since the Ipaq Glisten matches one of the 3g frequencies of T-mobile,

Will it work on T-mobiles 3g network if it is unlocked?
if not will it be able to make any calls at all?

Thanks
If one of the Frequencies match why wont it work?
How may have to match for it to work on T-mobile, and what would it be called a world phone?
If one of the Frequencies match why wont it work?
How many have to match for it to work on T-mobile, and why would it be called a world phone?

(sorry for the previous typos)