Temporal interocitor

The temporal interocitor was an experimental device to communicate across spacetime. It was created by the Fifth Doctor and Nyssa. She used it to talk to him while she was in 1320 Rhodes. (AUDIO: Renaissance of the Daleks)

After Nyssa left, the Doctor kept the interocitor. When Lucie Miller and Alex Campbell went travelling in Thailand, he gave them the interocitor so they could contact him if necessary. (AUDIO: Lucie Miller)

The Doctor later used similar technology to modify his companions' cell phones to allow them to not only be able to contact him wherever and whenever they were, but also other telecommunication devices from their original timezone. (TV: The End of the World)