Contacts RestApp and internal extension list

So, I finally got around to looking at this again, and worked out that for “Contacts” to work, a user needs to go to the UCP and add a group (in my test ‘suppliers’) and add a contact to that group. And it worked!
I can press the ‘book’ button on my Aastra/Mitel 6867 and a menu appears where I can pick the contact, great.

I also need the internal extension list to be available though, so a user can call someone without knowing their extension number. I can’t find a way to make this happen. Would anyone be so kind as to help out?