Did you just include your answer in your question?  Go to Custom Office, create a UDF to the Invoice Header and the Invoice Works file and make the source the telephone number from the customer master file.

If you use Sales Orders, the customer data source might not trigger when invoicing an SO.  In that case, flow from customer to SO to invoice (and work tables).

Also beware of multiple possible logical places to find the phone number (customer / ship-to address / confirm to contact...).  I always wondered about why Sage doesn't include a phone number for orders / invoices, but I guess the possible data sources are not simple to filter through.

If you use Sales Orders, the customer data source might not trigger when invoicing an SO.  In that case, flow from customer to SO to invoice (and work tables).

Also beware of multiple possible logical places to find the phone number (customer / ship-to address / confirm to contact...).  I always wondered about why Sage doesn't include a phone number for orders / invoices, but I guess the possible data sources are not simple to filter through.

I wonder that too Kevin, most shipping program has a phone number to be filled in. I believe invoice should have a phone number on it too.