Unless you are currently involved in communications with the IRS that were initiated by you in person or by the IRS via a letter, that person calling claiming to be the "IRS" is full of you know what.
The IRS does NOT initiate communications via phone calls. Scammers trying to steal your money or identity do. I have already had six people (including a tax pro I work with) receive these kind of calls.
If you want a little more reassurance, try googling the number the call came from (if the number is hidden it's DEFINETELY a scam). Chances are the google results will be full of people asking about the number, and MANY people identifying it as a scam.
If you want more reassurance, call the IRS (though this is pretty much a waste of time.)
I'm expecting the scammers to get more sophisticated and start sending letters, but that's for another post.
And BTW, that email from the IRS about a problem with your refund - that's a scam too.
UPDATE: Seven calls - including my sister! Now they are saying there are serious criminal charges. Still a total scam.
LAST UPDATE on call numbers: We are up to over three dozen people contacted in this way. We have also seen a couple of fake IRS letters. If you receive a letter from the IRS you should be careful to ensure that it is from the IRS. Most letters should be easy to verify based on information from your return being included on them, but if you have doubts, call the IRS at 1 (800) 829-1040 to verify. Don't use the number on the letter if you think it's a scam! You could also have a tax professional check it out. Also don't click on email links purporting to be from the IRS - these are also scams.