Your strongest tool against a questionable collector is a written validation request within 30 days. Here's how it works — paste your letter for a read first.
Once you request it in writing within 30 days, the collector must verify the debt — the amount, the original creditor, and that you actually owe it — and must pause collection until they do.
Send a short written request (keep a copy; certified mail is safest) asking them to validate the debt and identify the original creditor. Don't admit the debt is yours — just request validation.
If they can't verify it, they must stop collecting and can't report it. That's why a validation request is the first move on any debt you're unsure about.