Distress signal receiver

The distress signal receiver was supposedly a component in the Doctor's TARDIS that let the Doctor pick up SOSes from anyone, anywhere. Although he never knew who sent them, he would always answer them, no matter what. (DW: The Sontaran Experiment, Planet of Evil, The Creature from the Pit, Battlefield, Attack of the Cybermen, Terror of the Vervoids, Dalek)