I got the same message via my voicemail today. Somebody needs to shut this operation down. I believe it is a federal offense to harass by mail or phone. I have already reported scammers to the FBI and the FTC (Federal Trade Commission). This is a form of terrorism which should not be tolerated. Don't just be angry. Be active. Let's work together and shut these low lifes down!
I just received that same call and did answer because it was unavailable, but they did leave that message. I would think that a certified letter would have come from IRS if this were true. Not by phone call.