The bulletin was called into United Press International by the noted UPI reporter Merriman Smith. When he made the call, Smith was in the press "pool" car, in the presidential motorcade.
The car, Smith later wrote, was a "telephone company vehicle equipped with a mobile radio-telephone." He was in the car's front seat, with the telephone company driver, and Malcolm Kilduff, the acting White House Press Secretary for President Kennedy's trip to Texas--and Smith managed to commandeer the phone before any of the three other pool reporters, in the back seat of the car, could do so.
In 1964, Smith would win a Pulitzer Prize for his coverage of the assassination.
Later that day, it was Malcolm Kilduff who made the announcement to the press corps, in a nurses' classroom at Parkland Hospital, that President Kennedy had died.
(Teletype image, from the book Four Days: The Historical Record of the Death of President Kennedy, compiled by United Press International and American Heritage magazine, and published, in 1964, by the American Heritage Publishing Company.)