I also received letter 4883C a couple of days ago, asking to call [phone number removed] to verify my ID.
I had the same reservations regarding the possibility of this being a scam letter and 800 number. And like the person above, I could not find a reference to this 800 number on the IRS.gov web site.
So I called the regular IRS customer line 800 829 104 (see here: https://www.irs.gov/help/telephone-assistance).
The automated system is pretty convoluted, but following the instructions below I was able to talk to a human (see a related chat on the intuit site: https://ttlc.intuit.com/community/taxes/discussion/i-got-a-letter-from-irs-[product key removed]r-to...)
The agent confirmed [phone number removed] is the number to call for letter 4883c. Why they won't post this number on the irs web is a little baffling, but it is what it is.
