I need to show a customers id number on their invoice.

I have set up the fax field in create account to cater for this but I am now struggling to get it to show on the invoice. My intention would be to use the phone field (or email field) from the order table - meaning I would simply want to show the fax field instead of the phone. I thought it would be a simple matter of re-assigning the phone field in /classes/order.php but I cant seem to make it work.

Can anyone point me in the right direction please or have any suggestions?