The IRS does not initiate communications
with a threatening "cold call" to unsuspecting people.
Instead
The IRS always sends snailmail letters first!

There is a widespread IRS (or CRA) or "tax scam" being  practiced by scammers on both USA and Canadian residents.

USA tax payers, click on the "USA_Info" link, below, for lots of information and a reporting site.

USA_InfoUSA tax payers should read this:
http://www.irs.gov/uac/Newsroom/Phone-Scams-C ... 5-Filing-Season

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

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

Recent news story:
http://www.wltx.com/story/money/2015/03/12/fa ... -scam/70202814/

Congressional hearing:
http://www.nbcnews.com/business/taxes/tax-imp ... hearing-n341906
Canadian tax payers should read this:
http://www.cra-arc.gc.ca/nwsrm/lrts/2013/l131210-eng.html

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

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