My Dad is a retired officer from the IRS, and I confirmed this with him.  This call is bogus (even though it sounds somehat official) for the following reasons:
1.) You would have first been contacted by other means (mail, or other certified letter/package receipt previously)
2.)  If you had been contacted my other measures first, and then called; the person would identify themselves, as this would be a personal call, not an automated machine as this appears to be an auto-dialer of some kind, and perhaps they may even provide a case number or somthing.
3.) My dad, the retired IRS employee, has received this bogus call himself, and he has no lawsuit against him!