I keep getting reminders.

Foreigner's dunning starts with SMS and email, and generally does not call directly. But you can also make a phone call, but you can't pay for it immediately when you call someone else, so it's best to mail it. The simpler the format or content of the dialogue, the better, and the more the worse. For example:

Dear * * * *

This is a reminder that your storefront name account is due for payment. According to our agreement, a sum of money must be paid before the specified date. You can pay online, in our office or by calling the telephone number. Thank you!