Your question is kind of wide open since you don't give us details surrounding your process. I can help you organize your thinking, perhaps.
In general, if you need a notification, you must decide when you want it - and I don't mean "On the Due Date" but rather "When I launch my app" or "When I open a particular form" or something else, some other action.
You also imply that there is a certain day of the month on which this test should be implemented on a monthly basis. You also have mentioned a status, so what you want is a multi-fold test: (1) is it the day of the month to make calls? and (2) for each buyer, is the status other than "fully paid"? and (3) for unpaid statuses, is the current date later than the due date for a particular buyer or a set of buyers.
You don't describe your data either, so we might have trouble giving you specifics since we don't know your data names (if you have gotten that far in your design yet). You ALSO don't describe your procedures. So... suppose you have so many calls to make on a given day that you can't get to them all. Can that even happen? If so, what do you need to do about it?
All of these questions are rhetorical, because they are things YOU need to answer with regard to what you actually intended or needed to do.
As a final thought, Access is dumber than a box of rocks about anything other than making tables, queries, forms, reports, macros, and modules. YOU are the subject matter expert, so don't expect Access to implement anything specific. YOU would have to guide it through that process.
Start with a design phase of what you want to achieve. Then decide what information you need to implement that goal. THEN decide how to obtain or forward that needed information to that part of your app that needs to work with these due dates and statuses that might not be "fully paid" and other issues you will need.
