Only their phones work on their networks.
ATT, Cingular, and T-Mobile's GSM phones work with each other's networks, as does ATT and Cingular's old TDMA phones.
See? Now you see why I only have three listed. I like CDMA call quality better, but GSM phones are much neater and the SIM cards available in GSM phones make it easy to upgrade.
-US Nintendo Wii unmodded (waiting for modchip and install)
-US Playstation 2 v15 SCPH-77001 w/Crystal Chip Pro 2.0
-US V1.0 Xbox w/Xenium Ice and X2 custom BIOS