Space-time telegraph

The space-time telegraph was a device invented by the Doctor for the Brigadier to summon him to [[Earth] in the case of an emergency.

The Brigadier used it to get the Doctor to assist in investigating destroyed oil rigs. (DW: Revenge of the Cybermen, Terror of the Zygons)

The Brigadier used it again to bring the Doctor back for a training exercise, resulting in an argument between the two men. (ST: Hello Goodbye)

In 1996, Coldicott, on behalf of the United Nations, used it to get the Doctor to investigate the Remote. (EDA: Interference: Book One)

In the 21st century, The World Ecology Bureau asked UNIT to use the space-time telegraph to summon the Doctor to assist them investigating Alex Marlowe. (BFA: Hothouse)