You can't enter an
international phone number in the Mailing Address & Phone number section
of the Personal Interview. This phone number is for the IRS to call you with any questions about your return, but the IRS will not call an international phone
number.
You can enter the
phone number of a relative or friend in the U.S,, or a "dummy
number" (800-555-1212). This will allow you to complete your return, and
the IRS will identify your entry as a placeholder number - if they have
questions, they will mail them to you.
