Record both sides of the call

I've successfully implemented recording the voice through the microphone, but unable to record the downlink. How do I go about doing this? Here's the code I'm using -

audioManager = (AudioManager)getApplicationContext().getSystemService(Context.AUDIO_SERVICE);
    audioManager.setMode(AudioManager.MODE_IN_CALL);
    recorder = new MediaRecorder();
    recorder.setAudioSource(MediaRecorder.AudioSource.VOICE_COMMUNICATION);
    recorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);
    //recorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);
    recorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
    recorder.setOutputFile(getFilename());
    recorder.setOnErrorListener(errorListener);
    recorder.setOnInfoListener(infoListener);

    try {
        recorder.prepare();
        recorder.start();
    } catch (IllegalStateException e) {
        Log.e("REDORDING :: ", e.getMessage());
        e.printStackTrace();
    } catch (IOException e) {
        Log.e("REDORDING :: ", e.getMessage());
        e.printStackTrace();
    }