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Given that variable bit-rate VoIP can reveal words spoken from bitrate alone[1], I would be very cautious about their claims.

[1] https://www.schneier.com/blog/archives/2008/06/eavesdropping...

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I doubt their "signal processing" involves anything more than a microphone and a peak detector.

A more practical side channel attack would probably be through radiated emissions over the mains power. However if you're willing to go that far, a laser microphone is much more effective and simpler.

Depending on how wide is the peak detection window, they might send enough data to detect how long consecutive words are. I have a vague impression that I've read once that this, together with a language model, can be used to guess what's being spoken ~10% of the time or so.

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