Received a call  from IRS using this phone number. I asked him who he was trying to reach and he couldn't even tell me who he was looking for.

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.

https://www.irs.gov/uac/newsroom/irs-repeats-warning-about-phone-scams
http://money.cnn.com/2016/05/25/news/irs-impersonators-scam/

Called 20 minutes ago.  Did not answer. they left voicemail to call them back.  Number reported to the FBI.  https://tips.fbi.gov/

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

Called and left a robotic message about being sued and to call to get more info. right. blocking this number.