"Since the numbers on the keypad actually generated the same tones as those of a touch-tone telephone it occurred to us that instead of merely having a flashing light at the recipient`s end you could have a number appear corresponding to the number pressed on the keypad. If you could have one number appear you could have all numbers appear and in this way send a coded message. If the enemy was monitoring the airwaves all they would hear was a series of tones that would mean nothing."
"Talking To Vula
The Story of the Secret Underground Communications Network of Operation Vula"
Reminds me of the old days when nerds would transmit a digital "noise" over the air, you could record it with a cassette tape and it would actually be a playable game for a computer.
This story is something like that- and details actual spy craft. Fascinating read.