Anybody know how this is used? No info under help or in docs.
Cannot access it from a phone.

That is a network phonebook. It's only accessible from smt-i52xx phones.

OK but how do you access it from an IP phone?
the Phone Book button only shows what was entered directly by each phone.

Once you put entries in there you will get a network phone book option

I must be missing something, I have inserted listings under mmc 768 but on the phone sets when I press phone book I only get what I have entered at that set.