What will happen if I called back?

Probably nothing as long as you don't give them any information. It does, however, confirm you have a working phone number which may lead to more calls.
Best advice, get a call blocker ir don't answer calls from unknown numbers. Tere is NO LAW  requires you to answer any phone call.

I received a call yesterday at 10am (note post above). I called back maybe 20 or 30 times. They answered a few times but they started hanging up without saying anything. I waited an hour and called back again just to see what would happen by bugging them! At 3:00 I called back and they pick up and the guy repeatedly told me some things that he was going to do to my anus (to put it politely). So I waited a while and called back again. Finally when I got home I called again at 5:00. He answered and actually had a conversation with me asking why I kept calling back. I told him that he called me and I would not even know his number if he hadn't called me! I think we are handling this all wrong waiting for the government to do something about it like was mentioned above. What did President Obama say in his final speech? That it was up to the people to stand up and take care of things themselves! When you get a threatening call do not ignore it, call back and see what they want. If it is a scam and you can verify it on a site like this, put it on Facebook, ask a friend to call it, FLOOD THEIR LINES!!!!!! Have some friends call and straight out say this is a scam, at least we could make them have to react by at least getting a new phone or number. My thoughts are if I am tying up their time they are not talking to any unsuspecting victims, get as many people to call a scam number as you can!

this is a supposedly call from the IRS saying they are going to arrest me for fraud and to call this number, this needs to be stopped immediately and these people need to be arrested!!!  The IRS does not send out calls like this!!!