Request they they send you a debt validation letter (via the Postal Service, email and faxes do not count) as required by Federal Law.  Among the information that they are required to give you In that letter is the name and address of their company, the name of the original creditor, the amount owed and a statement that you have 30 days to contest the claim of debt.
If they refuse to send this to you via the Postal Service, they are not obeying the law and basically admitting that it is not a valid debt.  In that case, you can ignore them.