From http://www.irs.gov/uac/Tax-Scams-Consumer-Alerts
"Note that the IRS will never: 1) call to demand immediate payment, nor will the agency call about taxes owed without first having mailed you a bill; 2) demand that you pay taxes without giving you the opportunity to question or appeal the amount they say you owe; 3) require you to use a specific payment method for your taxes, such as a prepaid debit card; 4) ask for credit or debit card numbers over the phone; or 5) threaten to bring in local police or other law-enforcement groups to have you arrested for not paying."
They nearly got my gramma. Please don't call them back.
Recorded message. Female voice. Claims to be IRS stating they are getting ready to file lawsuit. Sick of this bogus crap. Great job again Federal government! Useless Do Not Call List.