SOS

An SOS was a type of distress call.

The Tenth Doctor used a spoon to send an Morse code SOS when he got trapped in a data core room on the planet Hurala. (PROSE: Prisoner of the Daleks)

A crewman told Captain Sacker about an SOS call, which asked for an archaeologist. (AUDIO: A Handful of Dust)

The Telian ambassador Dea was able to send an SOS while being held hostage on a Megallan prison ship. It was intercepted by the Time Lords, who sent K9 to trace the source and rescue whom he could. (PROSE: K9 and the Zeta Rescue)