“Numbers” stations on shortwave.
Since World War 1, foreign governments have been using shortwave to broadcast instructions to overseas operatives. Correct decoding of the messages relies on the possession of a codebook, or daily timepad.
This is achieved by transmitting a series of letters and numbers, sometimes using an automated synthesised voice, sometimes using Morse Code, and sometimes in a digital format.
Countries know to use or have used the “numbers” communications system includes: Cuba, Poland, Russia, North Korea, Israel, Czechoslovakia, Sweden, UK, and the USA.
Numbers stations have a great advantage over most forms of modern communications-they are completely anonymous. All an operative needs to do, is purchase a shortwave radio in the country in which they are located, and know the time and frequency of transmission.