call-agent.php

<?php
    header("content-type: text/xml");
    echo "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n";
    
    // This is the URL which the customer will be redirected to by th POST request to the Call Sid
?>
<Response>
    <Say>Thank you for holding. We are now connecting you to an agent.</Say>
    <Dial>
  	    <Number url="play-for-agent.xml">
  	    <?php echo $_GET['agent_phone_number'];?>
  	    </Number>
    </Dial>
</Response>

play-for-agent.php

<?php
    header("content-type: text/xml");
    echo "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n";
    
    // The caller will hear ringing until this TwiML completes, so keep this short.
?>
<Response>
    <Say>Incoming customer call.</Say>
</Response>

Redirect Customer Call

$ curl -XPOST https://api.twilio.com/2010-04-01/Accounts/{AccountSid}/Calls/{CallSid}.json \
    -d "Url=http://example.com/call-agent.php?agent_phone_number=%2b81345678901" \
    -d "Method=GET" \
    -u '{AccountSid}:{AuthToken}'

waiting.php

<?php
    header("content-type: text/xml");
    echo "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n";

    // The caller will loop through this TwiML until the agent makes a POST request to thier Call Sid

    if (!isset($_GET['repeat'])){
        // Do something to persist the Call SID and the fact that the call is waiting
    }
?>
<Response>
    <Say>We'll be with you shortly. Thank you for waiting.</Say>
    <Play>hold-music-1.mp3</Play>
    <Play>hold-music-2.mp3</Play>
    <Play>hold-music-3.mp3</Play>
    <Redirect method="GET">waiting.xml?repeat=true</Redirect>
</Response>