I am not sure about your code. This code works for me. Can you try it

function read_message_store_fax(){
  global $platform;
  $params = array(
      'dateFrom' => '2023-10-01T23:59:59.999Z',
      'dateTo' => '2023-10-20T23:59:59.999Z',
      'messageType' => 'Fax'
      );
  $resp = $platform->get('/account/~/extension/~/message-store', $params);
  // Limit API call to ~40 calls per minute to avoid exceeding API rate limit.
  $timePerApiCall = 2.1;
  foreach ($resp->json()->records as $record){
    if (isset($record->attachments)){
        foreach($record->attachments as $attachment){
            $fileName = $attachment->id . "_fax_attachment.";
            $fileNameExt = preg_split("/[/]/", $attachment->contentType, -1);
            $fileName .= $fileNameExt[1];
            try {
              $res = $platform->get($attachment->uri);
              $start = microtime(true) * 1000;
              file_put_contents($fileName, $res->raw());
              $end = microtime(true) * 1000;
              $time = ($end - $start);
              if($time < $timePerApiCall) {
                sleep($timePerApiCall-$time);
              }
            }catch (ApiException $e) {
              $message = $e->getMessage();
              print 'Expected HTTP Error: ' . $message . PHP_EOL;
            }
        }
    }
  }
}