I currently have a contract with AT&T on my 3GS iPhone. I still have the original iPhone my 3GS replaced. I would like to know if there is anything I need to do before leaving the U.S. and using my original iPhone out of the country. Can I simply buy a SIM card at my destination and pop it in? Or do I need to have this original iPhone unlocked by AT&T or jailbreak it myself? Thanks.