Scam. IRS does not call you and leave a voicemail. They'll come to your house, or your place of employment, but they don't call. Block this number, it's a widely publicized scam.
Those folks had the gall to leave a voicemail. "Just calling officially a final notice from IRS Internal Revenue Service. Reason of this call is to inform you that IRS is filing a lawsuit on your name because you have tried to do a fraud with the IRS..." Yeah right. Another number to block on my cell phone which is on the DNC list. Call originated from Jenkintown, PA.