Construye tu aplicación de mensajeria

<uses-permission android:name="android.permission.SEND_SMS"/>    

<application
...

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/str_para" />

    <EditText
        android:id="@+id/txtPhone"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="phone" >

        <requestFocus />
    </EditText>

    <TextView
        android:id="@+id/textView2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="@string/str_mensaje" />

    <EditText
        android:id="@+id/txtMsg"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:ems="10"
        android:inputType="textMultiLine" />

    <Button
        android:id="@+id/btnSend"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:text="@string/str_enviar" />

</LinearLayout>

    <string name="str_para">Para:</string>
    <string name="str_mensaje">Escribir mensaje:</string>
    <string name="str_enviar">Enviar</string>

import android.os.Bundle;
import android.app.Activity;
import android.app.PendingIntent;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.telephony.SmsManager;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends Activity {

 EditText txtPhone;
 EditText txtMsg;
 Button btnSend; 

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);

  txtPhone = ((EditText)findViewById(R.id.txtPhone ));
  txtMsg = ((EditText)findViewById(R.id.txtMsg ));
  btnSend = ((Button)findViewById(R.id.btnSend ));

  btnSend.setOnClickListener(
                new View.OnClickListener() {
                    public void onClick(View view) {               

                     PendingIntent sentIntent = PendingIntent.getBroadcast(getApplicationContext(), 0, new Intent("SMS_SENT"), 0);

                     registerReceiver(new BroadcastReceiver() {

                @Override
                public void onReceive(Context context, Intent intent) {
                 switch (getResultCode()){
                  case Activity.RESULT_OK:     
                   Toast.makeText(getApplicationContext(), "SMS enviado", Toast.LENGTH_SHORT).show();
                                     break;
                  case SmsManager.RESULT_ERROR_GENERIC_FAILURE:
                   Toast.makeText(getApplicationContext(), "No se pudo enviar SMS", Toast.LENGTH_SHORT).show();
                   break;
                  case SmsManager.RESULT_ERROR_NO_SERVICE:
                   Toast.makeText(getApplicationContext(), "Servicio no diponible", Toast.LENGTH_SHORT).show();
                   break;
                  case SmsManager.RESULT_ERROR_NULL_PDU:
                   Toast.makeText(getApplicationContext(), "PDU (Protocol Data Unit) es NULL", Toast.LENGTH_SHORT).show();
                   break;
                  case SmsManager.RESULT_ERROR_RADIO_OFF: 
                   Toast.makeText(getApplicationContext(), "Failed because radio was explicitly turned off", Toast.LENGTH_SHORT).show();
                   break;
                 }
                }       
                  }, new IntentFilter("SMS_SENT"));

                     SmsManager sms = SmsManager.getDefault(); 
                     if( txtPhone.getText().toString().length()> 0 &&
                         txtMsg.getText().toString().length()>0 )
                     {                      
                      sms.sendTextMessage( txtPhone.getText().toString() , null, txtMsg.getText().toString() , sentIntent, null); 
                     }
                     else
                     {
                      Toast.makeText(getApplicationContext(), "No se puede enviar, los datos son incorrectos", Toast.LENGTH_SHORT).show();
                     }
                    }});
 }

 @Override
 public boolean onCreateOptionsMenu(Menu menu) {
  // Inflate the menu; this adds items to the action bar if it is present.
  getMenuInflater().inflate(R.menu.main, menu);
  return true;
 }

}