Instead of a high frequency tone, just watermark the background noise or the speech pattern. You could watermark the background static, the voice samples, or ev

Instead of a high frequency tone, just watermark the background noise or the speech pattern. You could watermark the background static, the voice samples, or even the speech patterns. All you really need is something like 30 bits of data to identify a call as a Duplex call with very high probability, and I’m certain you can find a way to imprint that many bits into the frequency spectrum of your background noise.