SOS button

Evelyn Smythe gave SOS buttons capable of sending an alert to the TARDIS to Hebe Harrison (AUDIO: The Rotting Deep) and Elise Kaplan. (AUDIO: Maelstrom)

Hebe put the button on her mobile phone and pressed it on Albatross Alpha, leading the Sixth Doctor and Melanie Bush to come to her rescue. (AUDIO: The Rotting Deep) Elise pressed hers after trying to call Hebe to tell her about the Mindless Facility. (AUDIO: Maelstrom)

In an alternative timeline ruled by Patricia McBride, the Doctor gave an SOS button to Ron to use if Hebe needed help. He pressed it when she started remembering the true timeline. (AUDIO: Girl in a Bubble)