private function validateGovernment(string $vatNumber): bool
    {
        $prefix = strtoupper(substr($vatNumber, 0, 2));
        $number = (int) substr($vatNumber, 2, 3);

        // Government departments
        if ($prefix == 'GD') {
            return $number < 500;
        }

        // Health authorities
        if ($prefix == 'HA') {
            return $number > 499;
        }

        return false;
    }