title: calculate billing for twilio subaccount usage
body: <p>I need to get last month's usage for a specific twilio subaccount.
  I tried as specified in the official documentation, 
  I am getting a response when passing Master Account's Sid and Auth Token However, I can't get any details when passing the subaccount Credentials.
  Here's the code I tried out.</p>
  
  <pre><code>$sid    = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
  $token  = "your_auth_token";
  $twilio = new Client($sid, $token);
  
  $calls = $twilio-&gt;calls
                  -&gt;read(array("startTime" =&gt; new \DateTime('2018-1-15')));
  
  foreach ($calls as $record) {
      print($record-&gt;sid);
  }
  </code></pre>