Don't call them back. If they continue to bother you, track the number of calls. Debt collectors are subject to the provisions of the federal Fair Debt Collection Practices Act. Telemarketers are subject to the provisions of the federal Telephone Consumer Protection Act and must honor the National Do Not Call registry.
Which is all moot to foreign criminals impersonating Federal Agents. They are not debt collectors either.
IRS Scam:
The IRS does not need to sue anyone. They send you first class letters delivered by the Post Office. Several of them. Ignore those, and they then send certified letters that you need to sign for. Again delivered by the Post Office. Ignore those and they simply freeze your bank accounts and anything else you own. If they can do that, then why would they bother with a court case that might take them years before they get in front of a judge? They also do not bother arresting people for a few thousand dollars for the same reasons I have already given.
Official IRS Government site detailing this scam and providing links to report it:
http://www.irs.gov/uac/Newsroom/IRS-Warns-of-Pervasive-Telephone-Scam
If you feel you SSN has been compromised, in addition to putting a fraud alert at the 3 credit bureaus, you may want to notify the IRS. Here is the link, fill out the proper form (probably 14039):
http://www.irs.gov/Individuals/How-Do-You-Rep ... aud-Activity%3F