Start timer when user answers a call

Question

I'm making an app that should recognize when user answers outgoing call. The problem is that i have only 3 states in TelephonyManager, ringing, offhook and idle. When phone is dialing state is offhook even before the call is answered. How to recognize a moment when user answers on call? When I use default Dial app in my phone, the timer is shown when user answeres on a call. How to achieve this?

public class MyBroadcastReceiver extends BroadcastReceiver {
    private static int lastState = TelephonyManager.CALL_STATE_IDLE;
    private static Date callStartTime;
    private static boolean isIncoming;
    private static String savedNumber; 

    @Override
    public void onReceive(Context context, Intent intent) {

        //We listen to two intents.  The new outgoing call only tells us of an outgoing call.  We use it to get the number.
        if (intent.getAction().equals("android.intent.action.NEW_OUTGOING_CALL")) {
            savedNumber = intent.getExtras().getString("android.intent.extra.PHONE_NUMBER");
        }
        else{
            String stateStr = intent.getExtras().getString(TelephonyManager.EXTRA_STATE);
            String number = intent.getExtras().getString(TelephonyManager.EXTRA_INCOMING_NUMBER);
            int state = 0;
            if(stateStr.equals(TelephonyManager.EXTRA_STATE_IDLE)){
                state = TelephonyManager.CALL_STATE_IDLE;
            }
            else if(stateStr.equals(TelephonyManager.EXTRA_STATE_OFFHOOK)){
                state = TelephonyManager.CALL_STATE_OFFHOOK;
            }
            else if(stateStr.equals(TelephonyManager.EXTRA_STATE_RINGING)){
                state = TelephonyManager.CALL_STATE_RINGING;
            }
            onCallStateChanged(context, state, number);
        }
    }

    public void onCallStateChanged(Context context, int state, String number) {
        if(lastState == state){
            //No change, debounce extras
            return;
        }
        switch (state) {
            case TelephonyManager.CALL_STATE_RINGING:
                isIncoming = true;
                callStartTime = new Date();
                savedNumber = number;
                onIncomingCallStarted(context, number, callStartTime);
                break;
            case TelephonyManager.CALL_STATE_OFFHOOK:
                //Transition of ringing->offhook are pickups of incoming calls.  Nothing done on them
                if(lastState != TelephonyManager.CALL_STATE_RINGING){
                    isIncoming = false;
                    callStartTime = new Date();
                    onOutgoingCallStarted(context, savedNumber, callStartTime);
                    Toast.makeText(context, "Veza uspostavljena", Toast.LENGTH_SHORT).show();
                }
                break;
            case TelephonyManager.CALL_STATE_IDLE:
                //Went to idle-  this is the end of a call.  What type depends on previous state(s)
                if(lastState == TelephonyManager.CALL_STATE_RINGING){
                    //Ring but no pickup-  a miss
                    onMissedCall(context, savedNumber, callStartTime);
                }
                else if(isIncoming){
                    onIncomingCallEnded(context, savedNumber, callStartTime, new Date());
                }
                else{
                    onOutgoingCallEnded(context, savedNumber, callStartTime, new Date());
                    Toast.makeText(context, "Odbijen poziv", Toast.LENGTH_SHORT).show();
                }
                break;
        }
        lastState = state;
    }
}