public function getQueues($previous = null, $per_page = self::LIST_QUEUES_PER_PAGE)
    {
        $url = "projects/{$this->project_id}/queues";
        $params = array();
        if (!is_null($previous)) {
            $params['previous'] = $previous;
        }
        if ($per_page !== self::LIST_QUEUES_PER_PAGE) {
            $params['per_page'] = (int) $per_page;
        }
        $this->setJsonHeaders();
        return self::json_decode($this->apiCall(self::GET, $url, $params))->queues;
    }