Since World War I, a series of continuous shortwave radio transmissions with unknown origins have been broadcast around the world. The source and purpose of these encrypted transmissions is unknown, however, they are thought to hold secret communications sent from government intelligence services to field agents.
You can listen to a few here:
The nature of shortwave broadcasts is ideal for long-range travel, and very difficult to pin-point the origin of the transmission. The development of shortwave is attributed to Italian and British scientists during the 1920’s as a way to replace trans-atlantic telegraphy cables. Shortwave uses a phenomenon known as skywave or skip to bounce messages between the earths surface and the ionisphere. When transmitted at low angles, these messages can travel for thousands of miles, and be picked up by a receiver while keeping the message intact.
Numbers stations are thought to use one-time pad encryption, which has the advantages of being both information-theoretically secure, i.e., it cannot be broken even with unlimited computing power, and incredibly simple. The one-time pad was invented in 1882 by a Stanford Cryptographer. It is a key-based encryption technique, which uses modular addition to encrypt and decipher the code based on a key possessed by both the sender and receiver.
Numbers Stations saw their peak during the cold war. Many of these stations have been discontinued since, however, many numbers stations are still transmitted actively today. Due to their many security advantages over modern communication, it is very possible they are still in use today or are being used as decoys to mislead adversaries.