Telephone number entries in tax returns are formatted according to the 10-digit North American Numbering Plan, i.e., XXX-XXX-XXXX. Foreign telephone numbers begin with the country code, followed by the city code, and typically exceed ten digits in length.
To enter a foreign telephone number in TaxSlayer Pro, enter the last 10 digits. For your own purposes, you may wish to enter a return note with the full foreign telephone number. See here for information about creating a Return Note in TaxSlayer Pro.
Additional Information:
Desktop: How to Enter a Foreign Address
IRS: Common mistakes international filers make using Free File Fillable Forms