Good idea!!
We have calibration software that has built-in notices regarding the past due. The problems I have seen with this:
1. In general the people requiring the notifications are the people who are less 'organized' so to speak about these matters anyway. Thus, they can (and do) respond with "I didn't get any notice. Are you sure you sent one?"
2. Some of them looked so canned and boring, they tend to overlook them.
I have found sending an e-mail to them with a list serves the best. It gives me a clear trail to show what I did to follow-up. Then, I can forward that sent e-mail (to QA/ supervisor/ whoever) when it's time to escalate the issue.
As a suggestion... whatever you do, be very consistent and on-time or early. People will tend to want to avoid being harassed. So if they know on
every first day of the past due month they'll be getting a call...

they'll either work to get it in a little sooner; and won't be so irritated about being contacted. If they do get irritated, it's a simple matter of suggesting if you don't want to be contacted, please return xxxx/yyyyy/ whatever to you by the third week of the month.
