The set registration and replacement codes can be what ever you want them to and are defined in the system options form. If the extension is programmed for a 5304 you would lift the handset, dial the registration code + ext and hang up. If that fails, verify the code, set type on the DN and that there isn't a MAC already associated with the set. Or you could just manually enter the MAC in the user and devices form, I wouldn't suggest this for lots of phones as it would be annoying.