1 Make some api to transfer audio to another party of a call. As I know there is no api allowing to do it. But bluetooth headsets are working! Can we use a bluetooth chip as a proxy to send audio to be sent to another party into a baseband chip. Or maybe it is possible to modify some custom rom core and framework to get it without using bluetooth chip?.
2 Long long ago in pre-gprs era there was a technology called csd allowing to send data using in call. Bandwidth is about 12 kb/s. It was used to send faxes and data between customers, it was used to send sms and it was used to acqure inet access calling a special number given by the operator, like in modem times, but slower.
3 It is possible to get access to sound of the call using standart api
The idea is to create an app (good name for it is "Scrambler"), behawing in following way.
When customer calls somebody the app inserts a marker in the beginning of call. When another's app hears a marker, it enables csd (is it possiblewithout breaking call?).
Then eliptic curve Diffie-Hellman key exchange is going on, of course with mutual auth (using ecdsa) and digitally signing of each packet.
Then the apps record sound, encode it, then encrypt it (ctr mode is used because it is relatively packet loss tolerant) d send to another party, who will decrypt decode and play it.
It should prevent eavesdropping made by special services such as fbi, nsa and fss. But i doubt that it is possible, 12kb/s is very little bandwidth, especially if you add redundancy for error correction. But i heard that skype was working on old modems. What do you think about the idea?