Cingular Sim Card not compatible with other providers phone?

No you can't use your cingular sim in the phone which is locked with t-mobile until you unlock it ..You can easily unlock the phone through online providers like Unlock-Zone.com here they are rendering network unlock code for all mobile models at reliable cost .You first verify that whether the code is available for your mobile model ,country and network provider .If yes then pay for it and purchase the code then visit sites like MobileUnlockGuide.com to get free instruction about how to use that code and unlock your phone ...After unlocking the phone you can freely use it with any GSM sim ...