Asset Acceptance LLC collecting on something allready paid warren, Michigan

this company has been trying to collect on a debt that was paid off 6-18-02, and i have sent them copies of the pay off letter they just keep saying WELL our account number is different so you still owe , but i only had one account with this phone company and have called them and verified the paid account. the account they have is the same phone number as the one i paid 7 yrs ago . This company is holding me back from buying a house and i'm tired of fighting with them , but i am not backing down. They won't even call me back. Every time i call its hours of operation are closed no matter what time i cal its rediculous.