I answered and they said nothing. After a 10-15 seconds they hung up.

All they left on my answering machine was a horrible noise

They told me I was being sued from the IRS, if I did not call a number back. Can not remember the number.

Report the Calls to the real IRS here:
•report the incident to the Treasury Inspector General for Tax Administration at 800-366-4484.
•You can also file a complaint with the Federal Trade Commission at FTC.gov. Add "IRS Telephone Scam" to the comments in your complaint.

Do consider maybe getting a call blocking device or phone.. You can google them and many are around $60.00 or so.

http://www.irs.gov/uac/Tax-Scams-Consumer-Alerts
http://800notes.com/forum/ta-8c2f64bf2b91fa5/irs-warns-of-telephone-scam

This is a political robo-call from a US House of Representatives candidate in Kansas. Or at least that's the voice mail message.