Could be sloppy record-keeping or the hospital is just trying to cover all its bases.
Just because a voice on the telephone claims you owe a debt does not mean you owe it
If a voice on the telephone claims you owe a debt:
1. Demand the person give you his or her name, company name, and address
2. Demand a debt validation letter by US mail. E-mail no good.
3. Do not pay the collection agent or anyone else a DIME until the debt is validated.
They should have sent you some written documentation about your bills.