Old cingular sim card in new att phone?

Last year I finally upgraded my old flip phone to a smartphone. When the tech guy at the store got the sim card out, turns out it was an old Cingular sim card. We put it in the new one and it worked fine, but he gave me a new att one too just in case it started to "act funny." Anyway, fast forward a year and I need to now start using the voicemail feature, but it won't work. Is this because of the old Cingular card? Is it possible to put the new att card in and have the voicemail work (and not lose my numbers)? Or am I gonna have to sit on the phone with them for hours trying to figure this out?