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 always answered them. (DW: The Sontaran Experiment, Planet of Evil, The Creature from the Pit, Battlefield, Attack of the Cybermen, Terror of the Vervoids, Dalek, 42,The Curse of the Black Spot )

On at least three occasions, it was the psychic paper that received the distress signals. (DW: New Earth, Silence in the Library, Night Terrors)