999 (telephone number)

In the United Kingdom, 999 was the telephone number to contact the police in an emergency. (TV: For Tonight We Might Die)

After London was evacuated due to the appearance of dinosaurs, Sarah Jane Smith tried calling the police, but informed the Third Doctor that they weren't answering. (TV: Invasion of the Dinosaurs)

After the Sex Gas inhabiting Carys Fletcher killed Matt Stevens at the Nightspot, as witnessed by Banksy, owner of the nightclub, someone phoned the death in to 999, drawing the attention of Torchwood Three. (TV: Day One)

After the Tenth Doctor rescued Earth from and the Time Lords, and returned humanity to its original state, everyone began dialing 999. As a result, Sylvia Noble could not get through to emergency services to help her daughter, Donna Noble. (TV: The End of Time)

When Clyde Langer and Rani Chandra found that everyone else on Earth had vanished, Clyde tried calling 999, as well as 112, "the European one", but received no response. (TV: The Empty Planet)