收到错误类'TwilioRestClient'未找到

I have a message application for ios in which i wish to use twilio sms service. I need to make a webservice for this purpose, but i am getting an error:

Class 'TwilioRestClient' not found

Can anyone help me with it

<?php
    include_once "config.php";
    require "twilio-php-latest/Services/Twilio.php";


$phoneno = $_REQUEST['phoneno'];

$selectlist = mysql_query("select * from employee where phoneno = '".$phoneno."'");

//set authentication
$AccountSid = '***'; 
$Authtoken = '***'; 

//message values
$data = array(
 'From' => "***",
 'To' => "$phoneno", 
 'Body' => "Hello, Your guest has arrived at the reception"   
);

//new twilio rest client
$client = new Services_Twilio($AccountSid,$AuthToken);
$response = $client -> request("/$ApiVersion/AccountSid/SMS/Messages","POST",$data);

//check response for success or failure
if($response->IsError)
    {
        while($post = mysql_fetch_assoc($selectlist)) 
            {
                $data[] = $post;
            }
        header('Content-type: application/json');
        echo stripslashes(json_encode(array('error reading sms'=>'$response->ErrorMessage')));//echo" error reading sms: ($response->ErrorMessage)";
    }
else
    {
        header('Content-type: application/json');
        echo stripslashes(json_encode(array('sent message'=>'ResponseXml->Sid')));//echo "sent message: ($response->ResponseXml->Sid)";
    }
?>