It is hard to get hold of important people. They often have gatekeepers (e.g. secretaries). Never get on their bad side. And elevate their position in your mind. Treat them as the person’s partners. Go out of your way to appreciate such people (e.g. say thanks). Always treat them with dignity.
Never come off as aggressive to the gatekeeper. Definitely not on the first call.
Try different communication channels (phone, email, etc). Be persistent, but don’t irritate the gatekeeper. Expect it to take work. You’ll get better with practice.