So I'll set up the situation for you. I need to buy an iPhone 3G here in Boston. When I buy the phone, I will be changing my sister's account (she has an iPhone - standard $79.99 plan) to be included in a new family plan (with myself being the other user). I am an authorized user for the account, so I am allowed to do this. My question is, since the account is tied to a credit card other than my own, do I need to have that credit card with me to buy the phone?