protected function GetBillingCountryISOCode($oBillingAddress)
    {
        $sIsoCode = '';
        $oCountry = $oBillingAddress->GetFieldDataCountry();
        if (!is_null($oCountry)) {
            $oSystemCountry = $oCountry->GetFieldTCountry();
            if (!is_null($oSystemCountry)) {
                $sIsoCode = $oSystemCountry->fieldIsoCode2;
            }
        }

        return $sIsoCode;
    }