setVolumeControlStream android works weird

I've an issue setting the ringing volume in Android.

I'm working in a videocall app, so I need to make the phone sound when receive a call. The problem is, with the same code, if I make the phone sound in a outgoing call (for testing purposes) I'm able to manage the ringtone nice. Otherwise, in an incoming call, when I try to modify the volume, the audio sounds really weird.

I mean, there is an incoming call, the phone start the ringtone nice, but if I press on the key up volume, the ringing volume change first with a really low sound, and then changes again raising the volume up, and finally the ringtone keeps in the same volume than before (this change, from low to loud, happens in 1 second approx.).

I think the only difference between the outgoing call activity and the incoming call activity is that the first one is created tapping in a button, and the second one is created through a service which manages the incoming calls.

Here is my code to manage the ringing tone:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    //.....more code.....

    BroadcastReceiver receiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            AudioManager am = (AudioManager) getSystemService(Context.AUDIO_SERVICE);
            switch (am.getRingerMode()) {
            case AudioManager.RINGER_MODE_SILENT:
                log.debug("Ringing: Silent");
                break;
            case AudioManager.RINGER_MODE_VIBRATE:
                log.debug("Ringing: Vibrate");
                break;
            case AudioManager.RINGER_MODE_NORMAL:
                log.debug("Ringing: Normal");
                break;
            }
        }
    };
    IntentFilter filter = new IntentFilter(
            AudioManager.RINGER_MODE_CHANGED_ACTION);
    registerReceiver(receiver, filter);

    setVolumeControlStream(AudioManager.STREAM_RING);

    startRinging();
}

private Ringtone r;

private void startRinging() {
    if ((r != null) && (r.isPlaying())) {
        return;
    }

    Uri ringtone = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_RINGTONE);
    r = RingtoneManager.getRingtone(getApplicationContext(), ringtone);
    r.play();
}

Well...repeat, the same code works if I start the activity "by myself" (tested in an outgoing call activity), but if the activity is created through the service I cannot manage the ring stream properly. I don't know if that difference when create an activity is making the issue...

Here is the code from where I create the incoming call activity:

private final IncomingCallHandler incomingCallHandler = new IncomingCallHandler() {
    @Override
    public void onIncomingCall(IncomingCall call) {
        Class<?> activityClass = AppUtils
                .getClassByName(getString(R.string.incoming_call_package));
        if (activityClass == null)
            return;

        Intent i = new Intent(ctx, activityClass);
        i.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        i.putExtra(PARAM_INCOMING_CALL, call);
        startActivity(i);
    }
};

I need to get the activity using a string because is a configurable parameter, I cannot change that (and I don't think that is the problem).

Thanks a lot!! If someone need more information let me know!

Jorge.