Greyhound was a commonly used call sign in UNIT.
Simply Greyhound was used at various points by Alistair Gordon Lethbridge-Stewart, (TV: The Mind of Evil, The Claws of Axos, The Dæmons, The Time Monster, The Green Death, Battlefield) John Benton, (TV: Terror of the Autons) and Mike Yates. (TV: Day of the Daleks)
Greyhound Leader was used by Brigadier Lethbridge-Stewart. (TV: Robot, Terror of the Zygons) It was also once used by Benton when he answered on behalf of the Brig, (TV: Robot) and by Captain Clarke while the Brigadier and half of UNIT was away in Canada. (AUDIO: The Oseidon Adventure)
Greyhound One was also used by the Brigadier, (TV: The Green Death, Invasion of the Dinosaurs, Battlefield) as well as his daughter, Kate Stewart, once she was in charge. (AUDIO: Call to Arms, et al.)
Greyhound Two was used by both the Brigadier (TV: The Dæmons) and Captain Yates. (TV: Day of the Daleks) It was later used by Osgood, Kate Stewart's scientific advisor. (AUDIO: Call to Arms)
Yates used Greyhound Three whilst transporting the TARDIS to the Newton Institute. (TV: The Time Monster) Benton also used this sign. (TV: The Green Death)
Jo Grant once used Greyhound Four. (TV: The Green Death) It was also used by Benton. (HOMEVID: Wartime) When caught in the 1970s, Kate Stewart used Greyhound Four to contact Trap One, her father the Brigadier. (AUDIO: The Sacrifice of Jo Grant)
Doctor Martha Jones used Greyhound Six. (TV: The Sontaran Stratagem)
A UNIT soldier once used the call sign Greyhound Seven. (TV: The Mind of Evil)
In Donna's World, the call sign of Private Harris was Greyhound Fifteen. (TV: Turn Left)
Greyhound Sixteen was used by Steve Gray. (TV: The Sontaran Stratagem)
Greyhound Forty was used by Ross Jenkins (TV: The Poison Sky) and on one occasion by the Tenth Doctor using Jenkins' radio. (TV: The Sontaran Stratagem)