Mephistus

 was a teacher in 10th century Constantinople. He lost his job when he started hearing voices and people thought he was crazy. When he met the Fifth Doctor and Peri Brown, the Doctor determined that Mephistus was hearing a distress call from a spaceship in the future. The Doctor rescued the spaceship crew, stopping the voices, but left a radio transmitter with Mephistus so he could continue hearing the voices. (PROSE: Methuselah)