This script give your Google Voice a SIP Number (as well as a Skype number), which people can call without calling through real telephones. It also allows you to add phone number aliases (even International numbers) to your GV number so people in other areas can call your GV number as a local call, or at least without International calling fees.

For detailed help on this script check the my instructions on Google Docs:
https://docs.google.com/document/pub?id=1797lEe-jFF2e3HEchTVR5BP7mB6xATy4fYHmxOM0hk4

<?php
    //Main script settings
    $GV_NUM = "5555555555";
    $CALL_ERROR_MSG = "Sorry, but I could not connect to the Google Voice user. Please try again later";
    $HOLD_MUSIC = "http://www.jetcityorange.com/dtmf/DTMF-ringbackTone.mp3";

    $ALT_CALLER_ID = "";

    //When a call transfer failure occurs
    function callFailure($event) {
        say($CALL_ERROR_MSG);
    }

    //Transfering the call to GV
    if($currentCall->callerID == "") {
        transfer($GV_NUM, array(
        "playvalue" => $HOLD_MUSIC,
        "playrepeat" => '99',
        "timeout" => 120,
        "answerOnMedia" => true,
        "callerID" => $CALLER_ID,
        "onTimeout" => "callFailure",
        "onCallFailure" => "callFailure"));
    } else {
        transfer($GV_NUM, array(
        "playvalue" => $HOLD_MUSIC,
        "playrepeat" => '99',
        "timeout" => 120,
        "answerOnMedia" => true,
        "onTimeout" => "callFailure",
        "onCallFailure" => "callFailure"));
    }
?>