https://www.irs.gov/individuals/understanding-your-letter-4883c-or-6330c
What happens if I can't verify by phone?
We'll ask you to schedule an appointment and bring the documents listed above to your local IRS office to verify in person.
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
