If you call the number back, a guy with a heavy Indian accent says he's agent Mike Johnson from the IRS (although he can barely pronounce "Mike Johnson."). If you start laughing and ask him what his real name is, he gets upset and tells you to never call him again. TOTAL scam.
I am getting these calls also and it is robo voice leaving me a VM telling me to call them on a time sensitive issue. But they start the message before my voice mail is over so I never know who they are identifying themselves as. IRS will only contact you by mail is my understanding