How to detect when phone is answered or rejected

in your onReceive:

PhoneStateChangeListener pscl = new PhoneStateChangeListener();
TelephonyManager tm = (TelephonyManager)this.getSystemService(Context.TELEPHONY_SERVICE);
tm.listen(pscl, PhoneStateListener.LISTEN_CALL_STATE);

separate class:

private class PhoneStateChangeListener extends PhoneStateListener {
    public static boolean wasRinging;
    String LOG_TAG = "PhoneListener";
    @Override
    public void onCallStateChanged(int state, String incomingNumber) {
        switch(state){
            case TelephonyManager.CALL_STATE_RINGING:
                 Log.i(LOG_TAG, "RINGING");
                 wasRinging = true;
                 break;
            case TelephonyManager.CALL_STATE_OFFHOOK:
                 Log.i(LOG_TAG, "OFFHOOK");

                 if (!wasRinging) {
                     // Start your new activity
                 } else {
                     // Cancel your old activity
                 }

                 // this should be the last piece of code before the break
                 wasRinging = true;
                 break;
            case TelephonyManager.CALL_STATE_IDLE:
                 Log.i(LOG_TAG, "IDLE");
                 // this should be the last piece of code before the break
                 wasRinging = false;
                 break;
        }
    }
}

All you need to do is write some code to check if the previous state was 'ringing'. If the current state is idle and the previous state was ringing, they cancelled the call. If the current state is offhook and the previous state was ringing, they answered the call.

The above answer is completely wrong in case of outgoing calls. In Android there is no way by which one detect whether the call was actually answered (in case of outgoing calls). The moment you dial a number, the off_hook state is fired. This is one of the drawbacks of Android programming.