Android - Abrir WhatsApp desde Botón

public static void invokeMiActivity(Activity activity){

   String actionName= "nuestra.accion.nombreAccion";

   Intent intent = new Intent(actionName);

   activity.startActivity(intent);

}

public static void call(Activity activity){

   Intent intent = new Intent(Intent.ACTION_CALL);

   intent.setData(Uri.parse("tel:666-666-666"));

   activity.startActivity(intent);

}