As I understand you can keep your old AT&T plan until you either want to update your current plan or need to upgrade your phone. Please remember, no new AT&T handsets are being manufactured, unless you buy an unlocked phone.
Does anyone know - will I be forced to change from my AT&T plan to a Cingular one once my present contract expires or will Cingular let me go month to month with my old AT&T plan indefinently?

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