Dog whistle

Dog whistles emitted a high frequency noise. The 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 guards his whistle. One of them blew it and as it made no sound, declared it useless. (DW: The Armageddon Factor)

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)