Hi yes sure cingular sim is compatible in sidekick phone only after unlocking it from its present network provider .You can easily unlock your phone using network unlock code .You can purchase code from sites like Unlock-Zone.com they are rendering unlock code for all mobile models at reliable cost .You first check that whether the code is available for your mobile model , country and network provider.If yes then purchase the code and visit sites like MobileUnlockGuide.com to get free instruction about how to use that code and unlock your phone ....After unlocking the sidekick phone you can freely use it with any GSM sim .