Due vs Overdue

I overpaid last month of June. I do enquiries and the amount is due 01 August 2024. I got a call today and told the amount is overdue, I made 2 payments and the first one was counted for May. I stated I made 2 payments, how can I make a payment and it be backdated to the previous month??? She concluded by getting another colleague to call me, I'm seeing countless missed calls. What's wrong here, your accounting system???