The Better Business Bureau has put together a list of the top ten scams of 2015 and number one on the list (https://www.bbb.org/top10scams/ ) is the IRS scam where someone pretends to be from the Internal Revenue Service and calls you to inform you that you owe back taxes. The IRS never ever calls you regarding your income tax. They will only send you a letter via the United States Postal Service. They will also never email you. If you ever receive such a call, you can be reassured that this is a scam.
Feel free to slam the phone down, harass the person on the call, or do anything you like to the person claiming to be calling from the IRS. The real IRS still uses old fashioned snail mail in their efforts to contact you.