does cingular have a way of knowing what phone you're using? i'm on t-mo and thinking about switching to cingular. could i get one of the free phones from cingular, then just move the SIM card into my treo 600? i wouldn't mind having the cheap phone as a backup, for when i want to travel light. i would sign up for the mediaworks plan with the cheap phone, of course. would this work? would i be able to switch phones whenever i want?