Considering you configured your VoIP Drupal with one of the providers, here is the sample code snippet that sends "Hello world" SMS:

<?php
$number = '+1234567890123'; //Number to send SMS to. Must be in E.164 format.
$call = new VoipCall();
$call->setDestNumber($number);
$text = 'Hello from VoIP Drupal';
voip_text($text, $call);
?>

Pretty cool, right?