This past tax season I received several phone calls from clients concerned about highly threatening phone calls they received from people claiming to be from the criminal investigation unit of the IRS. I also received calls at my residence from these scam artists leaving threatening voicemail messages.
As you probably already know, you should never give your social security number or other personal information over the phone, especially from someone making threatening phone calls. As much as everyone tends to complain about the IRS, the IRS does not call up and make threatening calls. If you happen to be one of the targets of these scam calls, I would suggest hanging up immediately. The IRS does have a website to report such activities.