Ya sure you can use AT&T sim in the new go phone only if it is unlocked one else if the phone is locked to some other network provider then it wont accept AT&T or any other network sim until you unlock it .You can easily unlock your phone using sites like Unlocking4Free.com here they are generating network unlock code for some older mobile models at free of cost .Else if you want to unlock some other new model phone then visit sites like Unlock-Zone.com here they rendering network 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 pay for it and unlock your phone then freely use it with your AT&T sim .