The element "Seller contact telephone number" (BT-42) shall be provided.
(/ubl-invoice:Invoice/cac:AccountingSupplierParty/cac:Party/cac:Contact | /ubl-creditnote:CreditNote/cac:AccountingSupplierParty/cac:Party/cac:Contact)[$supplierCountryIsDE and $customerCountryIsDE]cbc:Telephone[boolean(normalize-space(.))]