Hi. You are referring to a 4883C Letter. You do not have to go in to the IRS to confirm your identity. You can call the toll-free IRS Identity Verification telephone number at 1-800-830-5084. Make sure to have a copy of the 4883C letter you received, a copy of your prior year tax return (if you filed one) and your most recently filed tax return (if you filed one), as well as any supporting documentation for each years return (such as W-2's, 1099's, Schedule C, Schedule F, etc) when you call.
Here is a link to the information found on the IRS.GOV site regarding this:
https://www.irs.gov/individuals/understanding-your-4883c-letter
