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

        if (!$this->_loadedCities) {
            $response = $this->call("getdidwwregions", array(
                "country_iso" => $this->getCountryIso(),
                "city_prefix" => $prefix
            ));


            $country = reset($response);
            $this->fromArray((array)$country);
            $this->_loadedCities = true;
        }
        return $this;
    }