I've done some searching but this is a pretty hard question for search engines. I need to know if an Android app itself can take full control of call sound I/O on calls to the PSTN.

I've found "background sound effects" apps e.g. to add traffic sounds to the background of a voice call, but I'm not sure this indicates what I'm interested in - the ability of an app to do ALL sound I/O on voice calls. And of course there are the call recorder apps which indicate that incoming voice call audio can be diverted and processed.

Examples where this would be useful:

  • realtime voice disguiser apps (shifts the pitch of your voice etc)
  • apps that detect when hold music is interrupted by a human so e.g. one can put the phone down and it will beep when a call center employee answers
  • apps which implement a DSP modem which can talk to a modem (or fax machine) on the PSTN, even if at low speeds like 4800-9600bps

The latter is my particular interest - being able to have an app which acts as a softmodem and can talk to modem-based devices on the PSTN. I'd imagine it might be technically possible given the CPU power of modern phones but I wonder if it's possible given Android's own capabilities.

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