Sometimes in a bad connectivity conditions I have a bit of trouble understanding the person on the other side of the phone. Since I'm using an Android device with a quad core CPU and additional DSPs, I would like to use that horsepower to enhance the clarity of the sound I hear during the call.
I am not aiming for a more accurate sound reproduction ("HD voice"), but clarity ("the other person can sound like a Martian, as long as I can understand him or her more clearly"). So it has to be an app which considers the human audio perception properties.
Another plus would be that it is an open-source application. It can require a rooted phone, which I think is going to be necessary anyway.