ADDENDUM:
Also, be sure the letter is legitimate. Although not as common as email scam, there have been some scams where fake letters were sent. If you phone the IRS to verify the letter, use their general phone number, not the one in the letter in case it's a scam.
IRS: 800-829-1040 (7AM-7 PM local time) Monday-Friday
Also see this FAQ, which explains the various types of IRS letters:
FAQ: I got a letter or notice from the IRS
https://ttlc.intuit.com/turbotax-support/en-us/help-article/internal-revenue-service/got-letter-noti...
