Could be any of a million reasons but one thing to try is get the IMEI number directly from the phone (usually dial *#06# on most phones, but may be Right * Left Left * Left * on your phone). Then verify the IMEI agrees with what may be written under the battery and then verify with Cingular that this agrees with what they have in their database. It is not unheard of for a clerk somewhere to transpose a number in your customer record or have you down in their records as having a different phone than you do. Vendors sometimes may accidentally switch boxes and stuff and only read off the box labels when they enter it in their records. Don't trust a code that Cingular just gives you without verifying directly that they have the correct IMEI.

Or, it could be something completely different.