OpenVPMS can send SMS reminders for unpaid invoices and counter sales.
For example, if an invoice is still unpaid 4 weeks after it was finalised, an SMS reminder can be sent out with the text:

Reminder: your invoice from Vets R Us on Tuesday 24/08/21 has an outstanding balance of $100.25. Contact us on (03) 12345678 if you have any issues. Please ignore this if you have recently made a payment.

This is fully configurable.

To do this:

1. The Practice must have SMS configured

2. Invoices and Counter Sales must have their Send Reminder flag ticked.  

This is enabled by default for new charges; historical charges must have this flag selected manually using the Enable Reminders button in Customers - Charges and Customers - Account

3. Customers must be able to receive SMS messages

This is done by selecting Allow SMS on their phone contact. If the customer has multiple phone contacts with Allow SMS set, then the preferred one will be used, and if none are preferred, then the first one created.

4. The Account Reminder Sender must be configured.

This automatically sends account reminders.

Reminders can be monitored in Reporting - Debtors - Reminders.