A numbers station is a type of shortwave radio station characterized by unusual broadcasts, often created by artificially generated voices reciting streams of numbers, words, letters, tunes or Morse code. They are transmitted in a wide variety of languages and the voices are usually female, although sometimes men's or children's voices are used.... more and more.
The Conet Project - Recordings of Shortwave Numbers Stations.
And also... How to Listen to Real Spy Broadcasts Right Now.
According to the notes of The Conet Project, which has compiled recordings of these transmissions, numbers stations have been reported since World War I. If accurate, this would count numbers stations among the earliest radio broadcasts. It has long been speculated, and was argued in court in one case, that these stations operate as a simple and foolproof method for government agencies to communicate with spies working undercover. According to this theory, the messages are encrypted with a one-time pad, to avoid any risk of decryption by the enemy. As evidence, numbers stations have changed details of their broadcasts or produced special, nonscheduled broadcasts coincident with extraordinary political events, such as the August Coup of 1991 in the Soviet Union.