The best way to verify that the number is official (mine was) is to Google:
site:irs.gov "[phone number removed]"
(Place the phone number given inside quotes.)
In my case, it turned up dozens of pages on the IRS site mentioning this number as official.
Totally agree with everyone else that the IRS isn't doing anyone any favors here by sending a letter that could so easily be a phishing attempt and then not helping you to at least verify the legitimacy of the number before calling to give out sensitive info. On top of that, when you do call the number, you're treated to a computer-synthesized voice that does little to allay the concern.
But the #1 way I knew this wasn't a scam was because, when I called that number, they made me hold for half an hour before someone picked up. And then it disconnected me. Yep, it's the IRS alright...
