How call PHP native function in Model/Service/Controller scope
You need to backslash the class to tell it to look in the global namespace like so:

public function validateVatNumber()
{
    $client = new \SoapClient("http://ec.europa.eu/taxation_customs/vies/checkVatService.wsdl");
}


You can read on the basics of PHP namespacing here: http://php.net/manual/en/language.namespaces.basics.php