If the VAT total amount in an invoice exists it MUST contain the suppliers VAT number.
ubl:Invoice
((cac:TaxTotal[cac:TaxSubtotal/cac:TaxCategory/cac:TaxScheme/cbc:ID = 'VAT']/cbc:TaxAmount != 0) and (cac:AccountingSupplierParty/cac:Party/cac:PartyTaxScheme/cbc:CompanyID) or (cac:TaxTotal[cac:TaxSubtotal/cac:TaxCategory/cac:TaxScheme/cbc:ID = 'VAT']/cbc:TaxAmount = 0) or not((cac:TaxTotal[cac:TaxSubtotal/cac:TaxCategory/cac:TaxScheme/cbc:ID = 'VAT'])))