Android send SMS automatically on button click

the easy way is to use SmsManager.Telephony.

You can use the build in Intent also:

 buttonSendSms_intent.setOnClickListener(new Button.OnClickListener(){

       @Override
       public void onClick(View arg0) {
        // TODO Auto-generated method stub

        String smsNumber = edittextSmsNumber.getText().toString();
        String smsText = edittextSmsText.getText().toString();

        Uri uri = Uri.parse("smsto:" + smsNumber);
        Intent intent = new Intent(Intent.ACTION_SENDTO, uri);
        intent.putExtra("sms_body", smsText);  
        startActivity(intent);
       }});

Try this

private static final String SMS_SENT_INTENT_FILTER = "com.yourapp.sms_send";
private static final String SMS_DELIVERED_INTENT_FILTER = "com.yourapp.sms_delivered";

String message = "hey, this is my message";

String phnNo = " " //preferable use complete international number

PendingIntent sentPI = PendingIntent.getBroadcast(this, 0, new Intent(
                SMS_SENT_INTENT_FILTER), 0);
PendingIntent deliveredPI = PendingIntent.getBroadcast(this, 0, new Intent(
                SMS_DELIVERED_INTENT_FILTER), 0);

SmsManager sms = SmsManager.getDefault();
sms.sendTextMessage(phnNo, null, message, sentPI, deliveredPI);

Try this code:

 String messageToSend = "this is a message";
 String number = "2121234567";

 SmsManager.getDefault().sendTextMessage(number, null, messageToSend, null,null);

With regards to the number, you need to enter the number as if you were calling it from the phone or sending an sms message in the normal manner.