Space-time telegraph

The space-time telegraph was a device given by the Doctor to the Brigadier, to be used to summon him to Earth in the case of an emergency.

History
The Brigadier used it to get the Fourth Doctor to assist in investigating destroyed oil rigs. (TV: 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. (PROSE: Hello Goodbye)

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

In 2001, the Brigadier used it to contact the Sixth Doctor for help in investigating the mysterious events in Turelhampton. (PROSE: The Shadow in the Glass)

In 2014, UNIT sent the Doctor a message to alert him to the threat posed by Rees. (AUDIO: The Screaming Skull) The message was received by the Sixth Doctor. (AUDIO: Second Sight)

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

After Elizabeth Klein met during his plan to rule all dimensions, the Seventh Doctor gave her a space-time telegraph to call him if she ever needed him. (AUDIO: Dominion)

The space-time telegraph eventually came to be stored in UNIT's Black Archive. When several Zygons infiltrated UNIT and Kate Lethbridge-Stewart activated the Black Archive's self destruct, the Eleventh Doctor, along with the Tenth and War Doctors, contacted her via the telegraph to try to stop her sacrificing London to save the world. Despite their warning, Kate felt she had no choice and ordered the telegraph shut down. (TV: The Day of the Doctor)

The Doctor had also given a space-time telegraph to Bob Lines. In 1989, he used it to summon the Sixth Doctor to deal with and the Usurians. (PROSE: Business Unusual)

In 2323, Georgette Gold found the Doctor recorded in the Dalek histories as well as a reference to "something called a space-time telegraph", which she recreated. Finding the time/space signature of the Doctor's TARDIS recorded in mil/ind/ent files, Georgette sent intelligent code into the ether as a distress call, successfully luring the Fourteenth Doctor to the Dalek Dome. (COMIC: )