Called my cell phone. Left a message saying the IRS is filing a lawsuit against me and directing me to call a number in the 202 area code. Not losing any sleep over this.
(1) IRS does not know my cell number.
(2) I'm the last person the IRS would sue. They love me. Being self-employed, I file Quarterly taxes and at any point in time, I run a favorable balance in the thousands of dollars. And I never take a refund. [Grin.]

Don't fall for this scam. I'm sure that when you call, they ask for your SS number and other identification.