In early December 1955, the phone rang at an air base in Colorado Springs. The officers on the watch floor of the Continental Air Defense Command (CONAD) - who were defending the skies above the US and Canada - stiffened.
The Cold War was in full swing and tensions were running high.
The command's director of operations Colonel Harry Shoup answered the call. On the other end was a child's voice asking: "Is this Santa Claus?"
According to the colonel's daughter Terri Van Keuren, now 75, her father initially thought it was a prank, and replied: "I'm the commander of the Combat Alert Center. Who's this?"
In response, the child started crying and asked if he was one of "Santa's helpers".
The colonel then decided to play along, replying that he was indeed Santa Claus and mustering a convincing "ho-ho-ho".
This surprise call started the nearly 70-year tradition of the Santa Tracker, which allows children around the world to track the whereabouts of Father Christmas via a livestream and a phone line answered by volunteers.
It is now run by CONAD's successor, the North American Aerospace Defence Command (NORAD).
But how did a child seemingly get the phone number of a colonel in the US air force?
The American department store Sears had printed an advert in a local newspaper telling children they could call Santa, Terri explains.
"They had printed one digit wrong in the phone number. And it was dad's top secret number."
Colonel Shoup called the phone company and asked for a new number for his office.
Meanwhile, the phone at CONAD was "ringing off the hook" and Colonel Shoup told his staff they were to answer the calls as Santa Claus.
In the story told by Terri, on 24 December that year her parents arrived at the base to deliver cookies to those on duty, and found the military establishment unusually festive.
A picture of a sleigh had been drawn by a map writer on plexiglass - which was used to mark where unidentified flying objects were located.
"Next thing they knew, dad was calling the radio station. 'This is Colonel Shoup, the commander of the Combat Alert Center in Colorado Springs. And we have an unidentified flying object. Why, it looks like a sleigh'," says Terri.
Terri, who lives in Castle Rock, Colorado, was six years old when her father became the "Santa Colonel". She says the NORAD Santa Tracker, which reaches millions of children around the world every year, is his "legacy".
NORAD's tracking of Santa is a military operation in itself beginning on 1 December.
Brigadier General Jocelyn Schermerhorn, a senior US military officer in Canada, tells Sky News how the day unfolds on Christmas Eve.
"We have about a thousand people come together to set up the operations centre that is used to track Santa and that allows anyone to call in to check on his whereabouts."
Volunteers are responsible for answering calls from tens of thousands of children around the world. In 2022, 78,000 calls were answered at Peterson Space Force Base.
For 10 years Terri was one of these volunteers. "I always wore a t-shirt that had a picture of my dad. It says: 'My dad's the Santa Colonel'."
What's next for the Santa Tracker? Terri says her father's festive story is so famous she's "had several requests to make a movie out of it".