Rest API - Production or Sandbox - Fax numbers not being returned

When using the Rest API functions( List Contacts, Retrieve a Contact) the fax numbers are not being returned.
I am using an email address with “List Contacts” function and a contact id with the “Retrieve a Contact” function.

Per the above links, the documentation shows that they should be in the response received back.

What am I missing❓


Fax numbers is an optional property try adding ?optional_properties=fax_numbers