There is a fairly common IRS or "tax scam" being practiced by scammers in both Canada and the USA.
USA tax payers should read this:
http://www.irs.gov/uac/Tax-Scams-Consumer-Alerts
Note particularly the links at that site, warning of the scam!
A shorter explanation is Scam #11, as shown at
http://phonehelp.2truth.com/facts.html
That site also shows how other 800notes visitors have reduced the number of telephone scams and telemarketing calls they receive. (That site also has a link to Amazon's phone blocker devices and apps, as well as a page showing various government sites that accept complaints or warn about scams.)