keep trying. unfortunately, the IRS does not have enough phone lines or staff (per CBS report) to answer all the calls coming in (general number 800-829-1040). another report said this year the IRS is receiving 4 times the volume than in prior years.
if you have been trying the phone number in the notice then
Another option is online verification
see this link
https://www.irs.gov/identity-theft-fraud-scams/identity-verification-for-irs-letter-recipients
if that doesn't work another option is to contact a local IRS office to see if you can set up an appointment.
