You can copy and paste this whole line into “DatabaseFieldSpecial” field in the label designer:
(case when phone1!='' then phone1 when phone2!='' then phone2 else cellphone end)
You can print label in customer section. Or in POS–>F3 Transactions–>More–>Print Custom Form. This label will be based on your transaction (customer and due date etc) or transaction items (product info and attributes). At end of the transaction, after payment window, re-print receipt area, there’s a “print custom form” button.