Where you hear about "numbers stations" here is what they are talking about. This video is of a hybrid station with the voice sending the message preamble and the data (sounds like a modem) sending the message.
The Perseus is a wonderful SDR receiver which captures this stuff real good.
Spy Stations
Moderator: Super Moderators
Spy Stations
"I predict future happiness for Americans if they can prevent the government from wasting the labors of the people under the pretense of taking care of them." - Thomas Jefferson