I got the voicemail also because I didn't know the number. Then I answered it and got an automated message saying 'The IRS is filing a lawsuit against me' and 'call now to get more information'. Total Scam!
Got over 10 calls from this number today and one was a robot voicemail saying I owed the IRS. DO NOT answer if they call it is a scam from someone claiming to be the IRS.