If for some reason, you need to send text messages to your clients cell phones, you should definitely have a look to TextMagic. They provide an easy API which allow you to send SMS to cell phones. Please note that the TextMagic service isn’t free.
The example below shows how easy it is to send a SMS to a cell phone using the TextMagic API:
// Include the TextMagic PHP lib require('textmagic-sms-api-php/TextMagicAPI.php'); // Set the username and password information $username = 'myusername'; $password = 'mypassword'; // Create a new instance of TM $router = new TextMagicAPI(array( 'username' => $username, 'password' => $password )); // Send a text message to '999-123-4567' $result = $router->send('Wake up!', array(9991234567), true); // result: Result is: Array ( [messages] => Array (  => 9991234567 ) [sent_text] => Wake up! [parts_count] => 1 )