gistfile1.html

<form id="google-voice" action="https://clients4.google.com/voice/embed/webButtonConnect" method="post">

  <input type="hidden" name="phone_call[buttonId]" value="<YOUR API KEY>" />
  <input type="hidden" name="phone_call[showCallerNumber]" value="1" />

  <ul>
    <li>
      <label>Your Name</label>
      <input type="text" name="phone_call[callerNumber]" />
    </li>
    <li>
      <label>Your Phone #</label>
      <input type="text" name="phone_call[callerNumber]" />
    </li>
    <li>
      <input type="submit" value="Call Me" />
    </li>
  </ul>

</form>