t-mo is generous giving the unlock code provided you're a subscriber for 30 days (ending the trial period and the conract starts) but at&t, it'll be hard and totally impossible for iphone for as they will never give it...

all phones that are quad band gsm will 100% work all over the world. no problem except slow internet. if you want 3g, only the nokia n8 covers all the 5 types 3g bands of the world...