Your callers are scammers. Under the FDCPA a debt collector is required to mail you a notice of validation within 5 business days of initial contact. NOT e-mail or fax. If they refuse, the case is closed. They are frauds attempting to collect on an invalid, time-barred or purely fictitious debt.
Another way to expose a fraud on the phone is to request their valid physical address. If they refuse to provide that information, they're frauds. The scammers are fond of using addresses that turn out to be UPS stores & mail drops. Verify any address that a supposed collector provides you. Google Maps is your friend. We've seen collectors from all over the country use addresses that turn out to be mail drops in strip malls, vacant lots, parking lots, bus stations & even sewage treatment plants. A debt collector is required, by law, to provide any supposed debtor with an actual physical address. Only the legitimate collection firms will do so. The scrubs working for the Junk Debt Buyers (JDBs) & the bogus collection outfits will not. They don't want to be found & they know that Cease & Desist letters will pour in. Not to mention the fact that they live in fear of aggressive consumer attorneys.
Research & know your rights under the FDCPA. It's not hard. Just Google it.
Be Safe. Be Aware.
Gunnar