Hi yes you can put your AT&T sim in your Blackberry curve 8900 mobile only if it is unlocked one ..Else if the phone is locked to some other network provider then until you unlock it you can't use it with your old sim ..You can easily unlock your phone using sites like MobileUnlockSolutions.com here they are rendering unlock code for all mobile models at reliable cost ..After unlocking your mobile you can freely use it with any GSM sim ...
