We plan to build some analytics on top of information about the user's participation in calls and meetings. We want to build those analytics in near real-time. To do that we need a new resource type representing any call itself with something like an attendee report. ie with the exact time when all participants joined or left the call. We need to have a change notification for that resource also. The desired schema is the following: when some user created a call notification about call creation received; when the second user accepts the call notification about update received with his joining time; when the third user joined the conference another update notification is received. and so on.