Trap One was a commonly used call sign in UNIT.
Trap One was used by Colonel Alan Mace during Operation Blue Sky, (TV: The Sontaran Stratagem/The Poison Sky) and following the Racnoss invasion of Earth in Donna's World. (TV: The Runaway Bride)
Behind the scenes
- The Trap One call sign appears to have been created to complement the Greyhound signs. "Trap" is the name given to a stall in which greyhounds are enclosed before the start of a race.