This concept was mentioned in this set of posts - https://forums.etarc.org/t/elizabeth-leighton-video-message-5/968/46
A numbers station is a shortwave radio station characterized by broadcasts of formatted numbers, which are believed to be addressed to intelligence officers operating in foreign countries. Most identified stations use speech synthesis to vocalize numbers, although digital modes, such as Phase-shift keying and Frequency-shift keying as well as Morse code transmissions are not uncommon. Most stations have set time schedules, or schedule patterns; however, other stations appear to broadcast at random times. Stations may or may not have set frequencies in the HF band
Numbers stations are anonymous, shortwave AM radio stations that broadcast messages at pre-set times, sometimes periodically and sometimes random, on specific frequencies. They’re notable for their unusual tone and content, as the stations can be silent for most hours of the day or week, then jump to life with a collection of artificial human voices, sounds, Morse code, short songs, or even nursery rhymes. They also broadcast in a number of different languages.
- https://en.wikipedia.org/wiki/Lincolnshire_Poacher_(numbers_station) (and a nice video - https://www.youtube.com/watch?v=QnXPqUU6fI0 ) - “The Lincolnshire Poacher” was a powerful shortwave numbers station that transmitted from Cyprus from the mid-1970s to June 2008.
- https://en.wikipedia.org/wiki/The_Conet_Project - The Conet Project: Recordings of Shortwave Numbers Stations is a four (later five) CD set of recordings of numbers stations and noise stations: shortwave (HF) radio stations of unknown origin
- https://archive.org/details/ird059 - good summary info and examples
- https://www.youtube.com/watch?v=TgF9ZlI_R-8 and https://www.reddit.com/r/UnresolvedMysteries/comments/55ninp/the_backwards_music_station/ - The signal has been determined to be originating from two sources, one in the United States and one in England.
- http://www.bbc.com/news/magazine-24910397 - nice overview of topic
- http://mentalfloss.com/article/65869/worlds-strangest-radio-broadcasts - easy to read article with examples