Hi all, I'm kind of clueless on the unlocking process and how the iPhone pricing works. I'd like to get my iPhone to run more like a Touch with Phone functionality - meaning I do not want the data plan (as wi-fi access is fine for my limited web use). If I was to buy a used iPhone, am I able to just insert my AT&T SIM card and go about using the iPhone? Or would I be forced to unlock the iPhone to pull this off, even though I'm still on AT&T? Thanks.