T-Mobile only uses the 1900 band, and Cingular uses 850 in almost all markets. There are a few major markets that Cingular uses the 1900 band, but that was during the time when GSM was first rolled out, and for a market like Chicago, 850 isn't wise for Cingular because there have been cases with Nextel that their phones interfered with emergency services frequencies. With that in mind, it's best if you get a dual-band phone from Cingular, or another GSM service that has the 850 band. If you go with a T-Mobile phone, then you need to get a quad-band phone, just to get the 850 band. Without the 850 & 1900 bands on the phone, the phone will only work in certain areas of the country.