"Lamarrs interest in gadgetry began in childhood, though she never pursued an engineering education. Her most influential brainchild was a method of covert radio communication called frequency hopping, which involves sending a message over many different frequencies, jumping between channels in an order known only to the sender and receiver. So if an adversary tried to jam the signal on a certain channel, it would be intercepted for only a moment." Excerpt from article in Science News: https://www.sciencenews.org/article/bombshell-hedy-lamarr-actress-tech-inventor
That sounds like a trunked radio system, which were used by ambulance companies to make it more difficult for people with scanners to follow their calls. Most of them now use cellphones, I think.