My wife heard on talk radio that the IRS had announced that if money was owed to them they would call in the future rather than coming to your home. That sounds like a scam; or, if true and the IRS actually plans to do this, I think that I would actually request a letter confirming the demand and attempt to follow-up on that by calling the local IRS service.
I have received 3 calls from this number today. I did call back once, and a man by the name of "John Marshall" (even told me to write down his name) claimed I was being sued by the IRS for tax fraud. When I questioned why no notice had been mailed, he said two notices had been mailed and read off my address. I guess he thought I was too dumb to realized that my name, phone number and address are all public record. I told him that I felt the call was fraudulent and he became very defensive. At that point I told him that I would be reporting the call to the IRS and told him to have a nice day. No one is ever going to notify you by PHONE that you are being sued. Legal documents have to be provided.