Is « Shepherd One » really used internationally by air traffic control for Pope's flight? CNN:


  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.


Wikimedia:


  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



Source

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.


Ghafla.com:


  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?
The FAA declared that as an official call sign for 2015-09-21 to 2015-10-15.



The order it cancels (.pdf here) had the bottom row filled out wrong (SHEPRD1 and "SHEPHERD ONE" were transposed, and "SHEPHERD" was typoed).

A document released on 2016-05-26 lists it in the "deleted special assignment" table (here).



So it's not a permanent reserved identifier in the US; but it might be recurring.

Wikipedia claims here (and here) that in Italy the Papal flights are named "Volo Papale ###", where the ### is a serial number (seemingly the number of times this Pope has flown).  I'll admit that I haven't been able to find anything akin to faa.gov for Italy, France, Australia, or the UK which would corroborate either "Volo Papale" or "Shepherd One".


  Typically, the Pope flies on a chartered Alitalia Jet. The Pope's flight is often nicknamed "Shepherd One" by the press, while the actual callsign is "Volo Papale" (papal flight, in Italian) followed by a serial number.