The Call Directory extension introduced in iOS 10 works like the Safari Content Blocker Extension that was introduced in iOS 9. Your extension has to supply a list of phone numbers (in safari’s extension, it was a json file), these numbers will be stored by the iOS, and on the event of an incoming call, the system checks the list provided by your extension( same like safari would check the rules specified by the json file before loading a website ), provided, your extension is active at that time.
So, using this feature for creating a Caller ID like app is not possible, because things are not dynamic.
Also there may be a limit on the number of phone numbers you can provide in the list, which hopefully be announced by the iOS 10 release.
