Same exact scenario happened to me - 3 calls from "unknown caller" and then they finally left a message yesterday, saying the same thing they did to you. Freaked me out at first - I know I don't owe the IRS anything, but of course they can make mistakes and cause you a lot of hassle. Now that I've read the posts on this website, I'm not going to even bother calling this obvious scam back.
Of course it's a SCAM! IRS does not call random #s nor leaves pre-recorded generic voice mails for no one inparticular stating they are about to file a lawsuit against you. This is total BS! These scammers can't even be bothered to make their scams look half-way believable!
It's a scam. There are at least two people behind this scam. One guy claims himself to be "officer Mark William, badge No. 2957" (with heavy Indian accent), and a woman.
Don't give them any personal or your financial information over the phone.