It makes sense that you wouldn't want to transmit messages only when your spies needed them, because that can leak information. So you constantly transmit rando

It makes sense that you wouldn't want to transmit messages only when your spies needed them, because that can leak information. So you constantly transmit random-looking information, and the spies have a special decrypter pad (either one time or multi use), and know that they can tune into the station at a certain time of day to receive instructions. If the instructions turn out to be jibberish after running it through the pad, then there are no new instructions. And you wouldn't want the transmission to require some esoteric equipment, whose mere ownership would indicate that the person is a spy. Using one numbers station, you could do one-way transmits of information to hundreds of different spies with commodity radi hardware who know when to tune in to get their slice of information.