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When I look in the Google play store (android apps market), I see that there are a lot of apps to make secure phone calls. However, all the apps I have seen so far all seem to use data packets over 3G/WiFi to encrypt the calls. Therefore it is not possible to make direct calls over the 'regular' phone network to a landline (or the app provides VOIP - and 3G/WiFi -for making a call to a landline).

So my questions are:

-Is there any app that actually encrypts the sound of your voice, and uses this 'encrypted sound' to make a regular voice call (not using data channel)? -Are there any open source implementations for Android for 'encrypting sound' (Systems like secure voice: http://en.wikipedia.org/wiki/Secure_voice)?

P.S. Feel free to correct me if I'm using wrong terms, I'm here to learn :)

closed as off-topic by Polynomial, schroeder, Xander, TildalWave, Rory Alsop Nov 28 '14 at 21:33

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  • How does one "encrypt sound"? I'm not sure that you understand how telephones work. They haven't used "sound" in many decades. – schroeder Nov 28 '14 at 17:33
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    @schroeder what he's saying is that the app should encrypt voice data, then convert that encrypted data to audio and send that audio over the phone network like a regular voice call, kinda like fax. – user42178 Nov 28 '14 at 18:05
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    @AndréDaniel I know that's what he meant. He's looking to make landline calls, though, which means he's looking for control over the telco that simply isn't possible. You'd have to have a custom modem on each end. – schroeder Nov 28 '14 at 18:32
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    @schroeder I think the app is supposed to be the modem, the telco just transmits what looks like to them a standard audio call (the audio being encrypted data). – user42178 Nov 28 '14 at 18:38
  • @AndréDaniel You said it way better then I could've expressed :) – user3231622 Nov 28 '14 at 18:46
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I don't think apps like that can exist on the current smartphone app markets, first because there are no APIs to directly control the modem to make calls and send an audio stream, and second because setting up a data link over an audio connection is possible but extremely slow and unreliable, too slow to pass any kind of encrypted VoIP traffic on it.

Technically what you say is possible and quite easy; it's just a matter of setting up a data link over a voice connection (amateur radio operators use AX.25), and once that's set up you can use whatever VoIP protocol you'd like on it.

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