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

I received a msg on Saturday morning from Nikki Johnson.  I called back and the 1st thing the man with a very heavy accent asked for was my ss#.  They said they were from the IRS.  I have spoken to the IRS in the past.  They ALWAYS give their ID# and name.  I was threatened unless I pay $4,000.00 I would be arrested Monday morning.  He again asked for my ss#.  I refused.  TOTAL SCAM!!!  Idiots won't ruin My day.

"I received a msg on Saturday morning "  That was your first hint.  What government agency works on a weekend or holiday?
"I called back and the 1st thing the man with a very heavy accent"  Calling back was a mistake, but your second hint should have been the heavy accent, probably Indian or Pakistan, but possibly Asian as well. "I was threatened unless I pay $4,000.00 I would be arrested Monday morning"  the IRS does not call people before sending letter out, and they do not arrest people over $4,000 in taxes.  Maybe 4 Million, not 4 thousand.  They have better and cheaper ways to collect it.
It is tax time and these sleazebags are looking to make a quick buck by stealing money from people:  http://www.irs.gov/uac/Newsroom/IRS-Warns-of-Pervasive-Telephone-Scam

As others have stated. Received the staunch call from one very busy "Officer Nicki Johnson with the IRS" stating that I, or my attorney of record had to contact her the "very second" I got the message, since it was of a grave "time sensitive" matter.

Nicki sounded like a computerized/robo voice. Caller ID showed Sausalito CA, they called at 8:00 AM Eastern DST, which means the call had to originate at 5:00 AM Pacific DST. Really? IRS worker working at 5 am???

Anyway, I have read, and heard through news reports of this scam. Obviously I did not, nor will I call this "officer" back for any reason.

First, the IRS doesn't have "officers." Secondly, IRS personnel talk at a very fast pace, this was way too slow and methodical, ergo computerize voice. Thirdly, they have other means of identifying themselves (all Federal Employees have) and this "officer" did not use it. Oh, and in case you spammers are monitoring this blog...news flash... it is not a badge number!

The number has been reported. I am sure by tomorrow they will use another spoofed number.

I do, however, urge any and all people who get this call not to engage with them in any manner . Your call will be re-routed from a U.S. line to a foreign scam mill and there may be serious consequences by doing this.

Additionally, immediately contact The Treasury Inspector General for Tax Administration (TIGTA) at:
www.tigta.gov or call 1-800-366-4484 to report this.
http://www.irs.gov/
http://www.irs.gov/uac/Newsroom/IRS-Warns-of-Pervasive-Telephone-Scam

Bottom line; say nothing to them, just hang up, and contact the above sites (number) or the local FBI office.

Good luck, and stay safe!