Dog whistle
From Tardis Wiki, the free Doctor Who reference
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)
When Sarah Jane Smith was given K9, she used a dog whistle to summon him. (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)
