What I would do is to first of all make copies of all of the correspondence that you have received including the envelope. Then do a google search (do not try to get this from irs.gov) for Form 4490 Proof of Claim 5 page Form and copy it down, but do not fill it in. Then I would send certified mail this form with a cover letter that tells them to fill out the form and get it back to you in 15 days sent to the last place that sent you a letter. Be sure to include your name and address and SSN on the cover letter and keep a copy of it and the Form 4490. After they do not respond (have not had one responded to yet) send a registered mail complaint to the universal postal union in Bern Switzerland stating that this agency is attempting to collect on a debt that they have refused to verify (cost about $25.) and are using the United States Mail to do it--mail fraud. You will need the correspondence from them including the envelope and use the copy of what you sent to the irs as an exhibit of what you sent to the irs.
(this is not legal or tax advice, etc.)