To complete and e-file your return, you must enter a U.S. phone number. Use the phone number of a relative or friend in the U.S,, or a "dummy number" (800-555-1212). The IRS will identify your entry as a placeholder number - if they have questions, they will mail them to you.
You can't enter an international phone number in the Mailing Address & Phone number section of the Personal Interview.
The IRS asks for your phone number in case they have a question about your return (this is rare). They will not, however, call an international phone number.