speaking in whistles

In "Whistled Languages: A Worldwide Inquiry on Human Whistled Speech" By Julien Meyer, there appears the following:

"... Whistled Spanish was used during Franco's dictatorship to avoid being understood by the Guardia Civil (i.e., the Spanish federal police).... Wam whistlers were even recruited by the Australian army during the Second World War to successfully evade Japanese radio spying, translating military messages into their language and then passing them through thrrough the radio using whistles (Nekitel 1992)."

I had been aware of the use of "code talkers" (those who spoke, I believe, Navajo) by the allies during World War II as a means of flummoxing the Japanese, but I was not aware of a whistling counterparts.