Signal

In 1903, Nikola Tesla intercepted the Mars signal with the Teslascope. Later, he signaled back. (TV: Nikola Tesla's Night of Terror)

In Pete's World, EarPods could be used to pick up signals from Venezuela to London. (TV: Rise of the Cybermen)

Following his defection to the Combined Galactic Resistance, Rusty transmitted a retreat signal to the Daleks, deceiving them into believing that the Aristotle had self-destructed. (TV: Into the Dalek)