Numbers stations (or number stations) are shortwave radio stations of uncertain origin. They generally broadcast artificially generated voices reading streams of numbers, words, letters (sometimes using a spelling alphabet), tunes or Morse code. 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. more on http://en.wikipedia.org/wiki/Numbers_station
Honestly, I thought such thing would only exists in something like LOST but it seems to be real.