Can i automatically send SMS (Without the user need to approve)

You can use this method to send an sms. If the sms is greater than 160 character then sendMultipartTextMessage is used.

private void sendSms(String phonenumber,String message, boolean isBinary)
{
    SmsManager manager = SmsManager.getDefault();

    PendingIntent piSend = PendingIntent.getBroadcast(this, 0, new Intent(SMS_SENT), 0);
    PendingIntent piDelivered = PendingIntent.getBroadcast(this, 0, new Intent(SMS_DELIVERED), 0);

    if(isBinary)
    {
            byte[] data = new byte[message.length()];

            for(int index=0; index<message.length() && index < MAX_SMS_MESSAGE_LENGTH; ++index)
            {
                    data[index] = (byte)message.charAt(index);
            }

            manager.sendDataMessage(phonenumber, null, (short) SMS_PORT, data,piSend, piDelivered);
    }
    else
    {
            int length = message.length();

            if(length > MAX_SMS_MESSAGE_LENGTH)
            {
                    ArrayList<String> messagelist = manager.divideMessage(message);

                    manager.sendMultipartTextMessage(phonenumber, null, messagelist, null, null);
            }
            else
            {
                    manager.sendTextMessage(phonenumber, null, message, piSend, piDelivered);
            }
    }
}

Update

piSend and piDelivered are Pending Intent They can trigger a broadcast when the method finish sending an SMS

Here is sample code for broadcast receiver

    private BroadcastReceiver receiver = new BroadcastReceiver() {

        @Override
        public void onReceive(Context context, Intent intent) {
            String message = null;

            switch (getResultCode()) {
            case Activity.RESULT_OK:
                message = "Message sent!";
                break;
            case SmsManager.RESULT_ERROR_GENERIC_FAILURE:
                message = "Error. Message not sent.";
                break;
            case SmsManager.RESULT_ERROR_NO_SERVICE:
                message = "Error: No service.";
                break;
            case SmsManager.RESULT_ERROR_NULL_PDU:
                message = "Error: Null PDU.";
                break;
            case SmsManager.RESULT_ERROR_RADIO_OFF:
                message = "Error: Radio off.";
                break;
            }

            AppMsg.makeText(SendMessagesWindow.this, message,
                    AppMsg.STYLE_CONFIRM).setLayoutGravity(Gravity.BOTTOM)
                    .show();
      }
  };

and you can register it using below line in your Activity

registerReceiver(receiver, new IntentFilter(SMS_SENT));  // SMS_SENT is a constant

Also don’t forget to unregister broadcast in onDestroy

@Override
protected void onDestroy() {
    unregisterReceiver(receiver);
    super.onDestroy();
}