Sample script to go directly to voicemail.

voicemail.php

<?php
require_once 'client.php'; //setup the Twilio rest client and assorted vars
$number = '1235556789'; //the number you're calling

$data = array('From' => $callerID,
              'To' => $number,
              'Url' => "http://twimlets.com/message?Message%5B0%5D=Voicemail&");

//place both calls
$call1 = $client->request("$version/Accounts/$accountSid/Calls", 'POST', $data);
$call2 = $client->request("$version/Accounts/$accountSid/Calls", 'POST', $data);

sleep(2); //allow the calls to actually start

//assuming the first call got through, the second was sent to voicemail
//so hangup the first call
$client->request("$version/Accounts/$accountSid/Calls/{$call1->ResponseXml->Call->Sid}", POST, array('Status' => 'completed'));