Numbers stations are shortwave radio stations of uncertain origin. They generally broadcast voices reading streams of numbers, words, letters (sometimes using a phonetic 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.

Evidence supports popular assumptions that the broadcasts are channels of communication used to send messages to spies....

http://en.wikipedia.org/wiki/Number_stations

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