Go to this IRS website for the 5071C letter - https://www.irs.gov/identity-theft-fraud-scams/identity-verification-for-irs-letter-recipients
If we can't verify your identity over the phone, we may ask you to schedule an appointment at your local IRS office to verify your identity in person.
You can contact your local IRS office. See this IRS website for local IRS offices - http://www.irs.gov/uac/Contact-Your-Local-IRS-Office-1 or call 1-844-545-5640 to set up an appointment
