If you have a T-Mobile, Cingular, or recent new M-mode type service with AT&T, then you have GSM service. GSM phones are not compatible with CDMA wireless.

In Canada Rogers and Fido are GSM.

GSM systems use encryption to make phone calls more secure. GSM operates in the 900-MHz and 1800-MHz bands in Europe and Asia, and in the 1900-MHz (1.9-GHz) band in the United States. When you see phones advertised as 900,1800,1900 capable they they will work in both Europe and North America under GSM.