There is a widespread IRS or "tax scam" being  practiced by scammers on both Canadian and USA residents.
USA tax payers should read this:
http://www.irs.gov/uac/Newsroom/IRS-Reiterate ... -Telephone-Scam

Short answer:
The IRS never initiates a contact via phone.
They always send a snailmail first.

Educational post:
This scam is also described as scam #11, here:
http://phonehelp.2truth.com/facts_rev.html

If you were contacted by an IRS impostor, you may file a report, after clicking on this link:
http://www.treasury.gov/tigta/contact_report_scam.shtml