That's because they have no idea who you are. They just have a computer dialing lists of numbers and they hope to get some poor, unsuspecting victim to believe them. It's a big SCAM. Just a bunch of thieves. Here's some info on them from the real IRS and a report of some of thieves who were caught.
They called me and left a voicemail using a robotic voice. All I heard was case file and please call back i don't know why I'm getting this spam call for