That is what I thought except for one thing; the black market jamming technology.
Such technology may be available for a while, but I would think that as monitoring increased, the availability of parts and facilities to manufacture and distribute jammers would be cut off.
I can see underground groups communicating quite effectively with ink and paper, or even spoken messages. In Frank Herbert's Dune series, people were specifically trained to memorize messages subconsciously; in other words, they would not be able to remember "at will" the message that they were carrying. In order to relay the message, they needed some sort of psychic stimulation, like a key word from a certain voice. Once activated, they would speak the message and then immediately forget what they had said (thanks to hypnosis).
I could even imagine messaging techniques being developed. I put a lot of faith in humans to navigate their way out from under crushing totalitarian rule. Ninja philosophy has always been a huge inspiration for me. The survivors will be the ultimate pragmatists.