Android :: Voice Calls Uses Its Own Audio Instead Of Build In Microphone

we're a group of students and trying to implement some kind of voice cryptography over a gsm connection (without modifing any gsm related codecs, we render pure audio which gets transmitted via gsm). Is it possible to initiate a voice call and transmit our generated audio signal (means bypassing the microphone as the primary audio source for that call)? I write this because I'm slightly worried after reading some posts on this list/group which states that the android.telphony APIs are private and can't be called from processes outside the the telephone subsystem.