Dog whistle

Dog whistles emitted a noise of so high a frequency, it was inaudible to most species. The Fourth Doctor used a dog whistle to summon K9 and to alert a Shrivenzale. (TV: The Ribos Operation) He used it again to summon K9 for him to track Romana (TV: The Stones of Blood). When she was captured by Count Grendel of Gracht, he sent K9 to his castle. (TV: The Androids of Tara)

When he was apprehended on Atrios, the Doctor gave the whistle to the Marshal. He in turn gave it to his aide Shapp, who blew it. As they could not hear it, the Marshal declared it useless. However, K9 was alerted by it. (TV: The Armageddon Factor)

Romana kept the whistle on her person. In his curiosity, Torvin blew the whistle when Romana showed him how. He was unaware of its function and never made the connection between it and the appearance of K9. (TV: The Creature from the Pit) The Doctor again used it while on Skonnos to summon K9. (TV: The Horns of Nimon)

Sarah Jane Smith used a dog whistle to summon K9. (TV: The Gift) Luke would use this much later to contact Mr Smith via Morse code (TV: The Man Who Never Was)

K9 Mark 2 gave dog whistles to his three teenage friends, Starkey, Darius Pike and Jorjie Turner. Unlike normal dog whistles, these operated more like mobile phones. (TV: Regeneration, Curse of Anubis, Aeolian)