Barbara speaks to Susan over the phone. (TV: "Sentence of Death"
On Marinus, the Arbitans had invented a telephone for communication across great distances. After Susan Foreman was kidnapped by Kala and Eyesen, she was made to call Barbara Wright, to inform her of the threat on her life should they not put an end to court proceedings in Millennius.
Kala and Eyesen themselves also used the telephone to give one another updates. Later, Barbara telephoned Investigator Tarron to inform him of Kala's guilt, and called him in. (TV: The Keys of Marinus)
Behind the scenes[]
Though none of the locals give it a name, Barbara calls it a telephone, while Susan distinguishes it from Earth technology, calling it "that phone thing" instead.
The prop for the Arbitan telephones appears to be an ordinary hot air brush.