Numbers stations are shortwave radio stations of uncertain origin. They generally broadcast voices reading streams of numbers, words, letters (sometimes using a radio alphabet), tunes or morse code.
The voices that can be heard on these stations are often mechanically generated. They are in a wide variety of languages, and the voices are usually women's, though sometimes men's or children's voices are used.
And NOW you can listen to many of the numbers stations (with video) on youtube.
You can watch a TV segment about them on youtube as well.
I was inspired by The Lincolnshire Poacher numbers station for my play. That station gets its name from the folk tune it uses. My play's fictional numbers station plays the folk tune Early One Morning.