When I first started working with Sage 100 I was baffled why phone numbers were not already on packing lists / invoices... but then I learned about how complicated the data flows can be depending on all the different ways transaction data can work (bill to customer, ship to customer, confirm to contact, ship to address...).

Try a UDF in SO / invoice header, then pass it from there to the work table.  This will only work for new transactions.