You would have to change all the phones if you have a shared (family) plan. If you have an individual plan, you won't have to upgrade all lines. Unfortunately shared plans can't be mixed between Cingular and former AT&T plans.
Its time for me to trade my nokia 6800 (had the thing for almost 3 years and its in pretty bad shape) so I decided to go to the cingular store. The problem is we have 3 cell phones in my household and they are all under at&t contracts. The cingular store clerk said that we would need to change all our phones and get a cingular plan. My question is do we need to change all our phones, or can I just buy a phone and put the at&t sim card in it and get it to work?

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