Cellphones are radios that work in various microwave bands. Shorter wavelengths have more trouble penetrating steel-reinforced concrete buildings. I don't know whether T-Mobile and Cingular are on the same band, but if they're not, the one with the longer wavelength will do better. Both are GSM phones, which use shorter wavelengths than the TDMA and CDMA systems employed by Sprint and Verizon, which is one of the reasons why Verizon will give better coverage than a GSM carrier.