As an ex-Cingular/ATT customer rep, that won't work. The way Cingular works is that everything is tied to the SIM. You can't have 2 SIM cards tied to one number. Additionally, once a SIM card has been used once,if you switch the SIM card ID number on the user's account, as soon as you do so, the previous SIM card becomes trash, and it can never be tied to a Cingular account ever again. I don't know how you can pull it off, since you can't have 2 phones tied to one phone number/account.