Welcome to the Make community!
According to the description below the Phone field, you need to insert the email address if phone is not empty.
Otherwise, you can insert 0000-000-0000 as the phone number.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!