If you listen to Shortwave you're already familiar with number stations. For the uninitiated, these are shortwave radio stations of unknown origin lacking Calls or ID that broadcast just sequences of numbers. The number are read clearly and occur in many languages. Voices can be male female or even computer generated.
Reports of Number Staitons date back to World War I and continue to this day. It is suspected that the broadcasts are coded messages used for communication with spies. Though no nation cops to this, I know of at least one trial where come cubans were prosecuted for espionage in relation to a numbers shortwave broadcast.
While the sequences are usually all numeric they can include words, or letters. They are sometimes apparently random, and other times organised. In the 90’s, amateur radio enthusiasts tracked the source of one number station to a US military base. The FCC refused to comment, duh.
The Conet Project: Recordings of Shortwave Numbers Stations is a four-CD set of recordings of numbers stations, mysterious shortwave radio stations of uncertain origin believed to be operated by government agencies to communicate with spies "in the field". The collection was released by England's Irdial-Discs record label in 1997, based on the work of number station enthusiast Akin Fernandez. You can download it all free here: http://irdial.hyperreal.org/