Both AT&T and T-Mobile use GSM at 850 and 1900 MHz in the US. European GSM phones use either 900 or 1800 MHz. So called world phones are usually tri or quad-band GSM that support either all four GSM bands or two of the bands in the target market and one band in the other (usually the higher frequency band).
Just for added confusion, there are also different 3G bands - the US uses 1900 MHz while most other countries use 2100 MHz (until recently 2100 MHz was restricted to the military in the US).
I used to be a member of AT&T's international wireless care team - so I've seen international phone issues
