You moved to prepaid. Postpaid care won't have access to your account via number since it's in a billing system they can't access.
Prepaid can't access your postpaid account for the same reason.
That being said, those reps suck. If you want your final postpaid bill, you contact standard care at 800-922-0204. From there, either give your account number or SSN to locate the account itself. EZ