I got the same letter, and was concerned. I was able to find the number referenced on this irs.gov page: https://www.irs.gov/irm/part25/irm_25-025-006r
I find it pretty ludicrous that the IRS expects potential identity fraud victims to call a hidden number sent to them in the mail, and give their social security number, 1040 information, etc.
They should use some form of mutual authentication. They prove to me, and I prove to them.
