Can I Search For Records With A Default Field With A Null Value?

Can I use Directories to search a Salesforce object for all records with an empty field?  For example - I want to find all contact records that do not have a cell phone number.

You cannot search for a field using a null or blank default value, unless it's a checkbox - in which case you would set the default value to "False".  Your Directories form will then return and display the records that do not have that checkbox selected.