If the IRS sent you a letter requesting verification, you have to either call them at the number provided on the letter or visit a local IRS office in your area. See details below and here Understand Your 4883C Letter
And you can visit this IRS website link to locate a local IRS office in your area - https://www.irs.gov/help/contact-your-local-irs-office
You should be able to search by your zip code and that will provide you their phone number and address. Most times, it will also state there if an appointment is required or not; and give you the option to make the appointment from there.
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