HI yes you can use AT&T sim card in sidekick only if it is unlocked one ..Else if the phone is locked to some other network then until you unlock it you can't use it with AT&T sim ....You can easily unlock your phone using sites like Unlocking4u.com here they are rendering unlock code for all model phones at reliable cost ....After unlocking your phone you can freely use it with any GSM sim ...
