You cannot interchange the SIM card just like that. If you want to use a Cingular network SIM card on a Tmobile phone, you need to unlock the device. You can unlock a Tmobile network locked phone safer and easier using unlock code. You can get the network unlock code either from the respective network provider or from any trusted vendor online. For example, you can visit Unlocking4Free.com to get the free unlock code for certain basic model phones. If your phone model is not available in this website, you can visit Unlock-Zone.com to get the accurate unlock code at affordable cost with an easy unlocking guide. Good luck.