Yeah... I've done that - the function is enabled, and I've always had call waiting, so it's not a function of the plan (in fact, they didn't change the plan at all when I got the iPhone, just added the data to the voice plan I already had). When I am on the phone, I hear the second call coming in, but can't switch to answer it because that "hold and answer" button is greyed out. That post you gave was informative but not exactly what's going on with me. I will stop in the AT&T store (because the bastards "forgot" to give me my apple stickers, the nerve!), and will try to resolve and report back what they say, in case someone else has a similar issue.