Folks

 I want to buy a iphone 3GS from the internet and put the sim((which is also a ATT sim which has a 3g logo) from my current phone (a RAZR) into it. . I have a ATT voice plan without the data. I just want to use the iphone as a basic phone and mp3 player without the internet. If it works, will ATT find out that I switched to iphone and begin charging me data usage fee (even I don't use data). Will I need to buy a unlocked iphone or a jailbroken one or both? What is firmware version 3.1.3? I heard that iphones with that version cannot be unlocked. I'm very confused.