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 1310 Rhodes. (AUDIO: Renaissance of the Daleks)
The Doctor kept the interocitor. When Lucie Miller and Alex Campbell went travelling in Thailand, the Eighth Doctor gave them the interocitor so they could contact him if necessary. (AUDIO: Lucie Miller)