Comment by wizzwizz4

Countermeasure: set up four microphones some distance apart, use autocorrelation to pinpoint the sound sources, and then isolate them, recovering the "masked" speech. The countercountermeasure would be to fully surround your mouth and vocal tract with an active noise cancelling system and then produce noise (to push whatever little sound gets through far below the noise floor: the signal is unpredictable enough that you can't use averaging techniques to recover it). The countercountercountermeasure would be to use a camera in the radio band to look at the vocal tract directly, using the phone as a light source, and recover the phonemes that way. The countercountercountercountermeasure would be to construct an isolated box… at which point you're no longer having a voice call in public: you have a portable privacy booth.