I received calls from other various numbers late last year with the same message, that the IRS is suing me. I blocked all of those numbers and was left alone for a few months. They must have gotten some new numbers to try again. This is a scam, and the IRS would never call about a potential lawsuit. Do not cal back. They will ask you for your personal information, but if you ask them for your case or how they can prove that they know who they are talking to, they won't know. They will also be very agitated and direct, trying to impose authority. Ignore any call that claims the IRS is suing you.
This appears to be a debt collector lying about being the IRS. They requested a person who has been giving my number on their accounts, so I get all the calls about them being delinquent.
To report a false IRS impersonator, fill out a form with the IRS:
https://www.treasury.gov/tigta/contact_report.shtml