Dog whistle

Dog whistles emitted a high frequency noise that was unhearable to most species. The Fourth Doctor used a dog whistle to summon K9 and to alert a Shrivenzale. (DW: The Ribos Operation) He used it again to summon K9 in order for him to track Romana (DW: The Stones of Blood) and again when she was captured by Count Grendel of Gracht, he sent K9 to his castle. (DW: 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 and, as its sound was unhearable to them, the Marshal declared it useless. However K9 was alerted by it. (DW: The Armageddon Factor)

Romana kept the whistle upon her person. In his curiosity, Torvin blew the whistle as Romana showed him how. He was unaware of its function and never made the connection between it and the appearance of K9. (DW: The Creature from the Pit)

Sarah Jane Smith used a dog whistle to summon K9. (SJA: The Gift)

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. (K9TV: Regeneration, Curse of Anubis, Aeolian)