protected function getRegionCodesForCountryCode($countryCallingCode)
    {
        if (!array_key_exists($countryCallingCode, $this->countryCallingCodeToRegionCodeMap)) {
            $regionCodes = null;
        } else {
            $regionCodes = $this->countryCallingCodeToRegionCodeMap[$countryCallingCode];
        }

        return ($regionCodes === null) ? array() : $regionCodes;
    }