who's to say they aren't doing both? They may not even be sending anything over the number station; these stations will continue on a schedule even when there is nothing to say and nobody is listening because it makes it harder to eek out a foothold in the event of a weakness in the encryption.

Even if the encryption is one-time pads, if you broadcast a bit every day then you don't warn the enemy that something's up by the fact that you're transmitting at all.