public function loadPSTNNetworks($networkPrefix = NULL)
    {
        if (!$this->getCountryIso()) {
            throw new CountryException("ISO code is undefined");
        }
        if (!$this->_loadedNetworks) {

            $response = $this->call("getdidwwpstnrates", array(
                "country_iso" => $this->getCountryIso(),
                "pstn_prefix" => $networkPrefix
            ));
            $country = reset($response);
            $this->fromArray((array)$country);
            $this->_loadedNetworks = true;
        }
        return $this;
    }