Can I enter a foreign telephone number? (I don't have a U.S. number, and I can't continue past the "mailing address" page.)

You are not able to enter a foreign phone number.  You can either skip it or enter a friends US phone number or a "dummy" US phone 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.   Besides, the IRS never calls anyone, they send everything by mail.