From wikipedia:
“A numbers station is a shortwave radio stationcharacterized 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.
The first known use of numbers stations was during World War I, and possibly the first listener was Archduke Anton of Austria. The numbers were transmitted in Morse code. Numbers stations operated mainly during the Cold War, yet many continue to operate and some have even switched operators.”