This page says that you can't use the app unless you have a US telephone number.
www.irs.gov
But there is a phone number you can call if you don't have the i.d. information they require.
Identity and Tax Return Verification Service | Internal Revenue Service
Use the identity verification (ID Verify) service if you received an IRS 5071C letter, 5747C letter or 5447C letter.
But there is a phone number you can call if you don't have the i.d. information they require.