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.
To accomplish this, you can create a formula field on the object being searched that evaluates the field in question and returns a value for any record with a NULL or empty value. This new field can be searched with Directories.