"Seller contact telephone number" (BT-42) should contain a valid telephone number. A valid telephone should consist of 3 digits minimum.
(/ubl-invoice:Invoice/cac:AccountingSupplierParty/cac:Party/cac:Contact | /ubl-creditnote:CreditNote/cac:AccountingSupplierParty/cac:Party/cac:Contact)[$supplierCountryIsDE and $customerCountryIsDE]matches(normalize-space(cbc:Telephone), $XR-TELEPHONE-REGEX)