Didn't answer; they left a message. Robocall with a robot voice, claimed to be from the IRS, and they had information that I had been withholding information to avoid taxes, and that I needed to return the call immediately to avoid criminal charges. Clearly a scam, because 1) I don't owe any taxes, and 2) the IRS doesn't operate this way. They will send mail first, giving the taxpayer a chance to explain or simply to pay without explanation. Then they'll follow that up with another letter (registered?). Think about it -- when was the last time you even entered your phone number on your 1040? You don't. There's not even space for it. Turbotax and other services will ask for it, but it's for their information, not for the IRS.