Welcome to the Make community!
You can use the built-in function ifempty to default to a non-empty value
e.g.:
{{ ifempty(1.phone; "00000000") }}
For more information, see the function documentation in the Help Center.
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!