You'll have to have all mailboxes on each system you need dial by name by.
Or, upgrade to a centralized voice mail,
Or, incorporate the new Microsoft Exchange stuff -really cool, voice activated, call me if you want a demo.
Or, pick up a used voice mail system for that express purpose. You could even use an old DOS machine for that. Cheap solution but it's another data base you'd have to maintain. If it crashes you pick up another one.
Ralph