Yes you use Old AT&T sim card in an I-phone,before that you need to check whether your i-phone is an unlocked one....To check if its locked or unlocked one,insert the sim in the phone ,if it will display the message Invalid Sim or else enter unlock code ,it's a locked phone,otherwise if its accepts the sim card hence its an unlocked phone.To Unlock a locked I-phone visit the site
Prounlocking.com/apple-iphone/rs8wp9/ get remote unlock service and unlock it easily,then use any gsm network sim card.