public function getProperties($billingAccount)
    {
        if (!$billingAccount)
            throw new BadMethodCallException('Parameter $billingAccount is missing.');
        try {
            $r = $this->get('telephony/' . $billingAccount)->send();
        } catch (\Exception $e) {
            throw new TelephonyException($e->getMessage(), $e->getCode(), $e);
        }
        return $r->getBody(true);
    }