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 Fourth 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 Eighth 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)