Sure they can customize their view. You just can't prevent them from adding extensions that "belong" to another business unit if they wanted to. When I click on add to phonebook, I can see/search ALL extensions on the systems and add any of them to my switchboard. You can prevent a user from seeing who the other extensions are talking to, but they would still see whether or not they're on the phone, and presence status.
You mentioned in your first post that you "didn't want my other subsidiaries to know each other presence". What I'm saying is that you can't prevent users from one subsidiary, from adding an extension of another subsidiary, and thus having the ability to see their presence.
Let's say that Company A uses extensions in the 1xx range. Company B uses extensions in the 2xx range.
Extension 101 logs into their switchboard and clicks on "Add Phonebook Entries", "Create Phonebook Entry", then "System Extension". When extension 101 clicks on the search icon, he will see all 1xx extensions and all 2xx extensions. If extension 101 only adds 1xx entries in his phonebook, then sure that is all he will see in his switchboard. But the point here is that you cannot prevent him from adding 2xx extensions if he wanted to.
Now you can prevent users in the 1xx range from seeing the caller id/number that extensions in the 2xx are talking to, but keep in mind that even doing this, users in the 1xx range, if they add users in the 2xx range to their switchboard, would be able to tell if extension 2xx is on a call or not.
I hope that helps clarify things for you.