There has been a scam going around where the caller claims to be with the IRS and that we (yes, I work for the IRS) are going to file a lawsuit to recover taxes due. This is a scam! The IRS does not make calls like this; the IRS would send at least 4 letters before any action will be taken and we do not need to file a lawsuit to recover back taxes. I have received about 4 of these calls - there is a possible follow-up call that purports to be from a sheriff who is on the way to serve the papers (I received 2 of these calls), ignore this one also.
If you receive any call of this type, you can report it to the Treasury Inspector General for Tax Administration at: TIGTA.gov. There is a bright red link for reporting these calls.
Worst one I know about was the taxpayer was on a landline with the fraudulent IRS agent and she received a call from the 'sheriff' - she tried to get her bank to do a money transfer but they told her it was a scam. She was completely freaked with the sheriff speaking gruffly, the 'IRS agent' yelling at her, and the bank refusing to do the transfer. She finally hung up on everyone.