I received a similar message, almost identical to the one you stated. Did they return your call and did you report that to the IRS? I am reporting this to IRS.
This is a well known IRS scam perpetrated from Pakistan (these scammers use Voice over IP phones). You can usually tell by the thick accents. Sometimes they use spoofed numbers to seem real.
Please read http://www.treasury.gov/tigta/press/press_tigta-2014-03.htm.
All others who read this, please do not fall for this. The IRS never calls directly to open a case.
Scammed me our of 100$ said I had yo pay 5000$ in back taxes if not they'd send a warrent out for my arrest I don't owe any back taxes and when they called they kept me on the phone for 2 hours I'm out if that money now and can't get it back they wanted it in a money pack from Walgreens and. Unfortunately they cantteace it now