20,000 Hertz, a professionally produced podcast on sound (sound design, editing, movie and TV sounds, jingles, natural and manmade sound, etc.) has just released their most recent episode titled "The Buzzer", about number stations and the like.
Since World War I, countries around the world have been broadcasting mysterious numerical messages via shortwave radio. Though concrete evidence is hard to come by, the general consensus is that these coded messages are meant for undercover agents operating abroad. And one particular Russian station