The President flies on Air Force One. The Pope flies on "Shepherd One." That's the Federal Aviation-approved call sign for the Boeing 777 in which he'll be traveling around the United States, according to an official from American Airlines, which will be operating the plane.
Shepherd One is the international call sign of any aircraft carrying the pope onboard. The airline which is used for such transportation is usually Alitalia
Perdue site ('Cradle of Astronauts'):
Flying Pope Francis – Purdue alum honored to fly 'Shepherd One'
From Crux Now catholic press:
The plane isn’t really called “Shepherd One.” People in the United States call it that, but the phrase is a media conceit rather than an actual call sign.
Did you know it’s Americans who baptized the plane carrying the Pope ‘Shepherd One’? Well, their very own presidential jet is called ‘Air Force One’ so they thought since the Pope is a shepherd leading the Catholic Church, his plane should be called ‘Shepherd One’.
Question: Does ATC around the world, starting with Italy and the US, uses the specific call sign Shepherd One?