Sooner or later every Shortwave Listener will hear them, the Number Stations: rarely letter-groups are read by a synthezised male or female voice, are keyed in Morse code or are sent in digital mode.
All transmissions start with an identifier/call, usually a 3-figure group, repeated several times. The following figure-groups specify the decryption key/group-count of the message which ends, for example, with a series of “Zero”s (or “out” if spoken).
Every network has its own “fingerprint”: transmission schedule, modulation and format are typical for a specific network and will help the listener assigning it to a certain country. An increasing number of utility listener are into the hunt for Number Stations.