Oulook Contact email address format

In our Exchange 5.5 Server we have a Public Contacts folder that all users can access. Viewing this Contact list with Outlook 2000, some local users (in our domain) have their email address listed as their name ie “Sam Smith”, while others are listed as their email address ie “sam@home.com”. Why is this and how can I set it to display the full email address for all users?