I'm an American living abroad and will be returning to the US for a week during the holidays. I have a factory unlocked iPhone 4 and still have my old AT&T microSIM from when I lived in the US. I was wondering whether I could use my old AT&T microSIM and simply purchase a $25 GoPhone refill card to get voice, text and data. Or do I need to get a GoPhone-specific SIM card? I know there's been a lot of discussion about what can and cannot be done on an iPhone through GoPhone -- I don't necessarily want to rehash all of that: I just have the very specific questions above about whether the GoPhone plan requires a GoPhone-specific SIM and whether I can get a GoPhone plan up and running from just the refill card.