I was reading Merriman Smith's 1962 book The Good New Days in which he describes the famous White House operators. When the president says "Get me the Secretary of Defense," they find him, no matter where on the planet he is. From the book:
Sometimes the president, after starting his business, will say, "Did I get you at home?"
And the Cabinet member answers, "No, Mr. President, I'm in
This phenomenon is familiar to all of us now because we call people on their cell phones. We never know where we'll find them. Presidents were the only ones on the planet almost who experienced this for several decades.