PHP implementation of sip2pstn which allows outgoing calls via Twilio from your SIP registered phone. Within Twilio, programmable voice, SIP Domain, you should point the Voice URL at this file (on your server).

sip2pstn.php

<?php
// should be configured in twilio under Programmable Voice > SIP Domain > Voice Config
// http://yourdomain.com/sip2pstn.php?callerId=%2B<yourTwilioNumberIncludingCountryCode>


// to adapt this for other countries
// - change the +44 to your own country code and 
// - change the ringTone from 'uk' to your country (us, de, etc)
// this is not currently setup for USA 10 digit dialing (ie you should dial 001 or +1 or 1 before the 10 digit US number.
$localCountryCode="+44";
$localRingTone="uk";

$from = $_REQUEST['callerId']; // one of your twilio numbers
$to=$_REQUEST['To'];
if (empty($to)) {
    header("HTTP/1.1 500 Internal Server Error");
    echo 'Missing "To" parameter.';
    die;
}

// extract To number from request
preg_match('/^sip:([+]?[0-9]{10,14})@/', $to, $number_matches, PREG_OFFSET_CAPTURE);

if (sizeof($number_matches) != 2) {
    header("HTTP/1.1 500 Internal Server Error");
    die;
}
else if ($number_matches[1][0][0] == '0') {
    if ($number_matches[1][0][1] == '0') { // 00 prefix, replace with +
        $number = '+' . substr($number_matches[1][0], 2);
    }
    else { // 0 prefix, replace 0 with +CountryCode
        $number = $localCountryCode . substr($number_matches[1][0], 1);
    }
}
else if ($number_matches[1][0][0] == '+') { // already formatted +number
    $number = $number_matches[1][0]; 
}
else { // starts with 1-9, prepend +
    $number = '+' . $number_matches[1][0];
}

header("content-type: text/xml");
echo "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n";
?>
<Response>
    <Dial ringTone="<?=$localRingTone?>" answerOnBridge="true" callerId="<?=$from?>" record="do-not-record">
        <Number><?=$number?></Number>
    </Dial>
</Response>