Continuous Harassment Over Paid Invoice

I've been receiving calls from your company that I'm owing R1840 of which I paid last year in September, I spoke to one of your agents about this matter and told me they will allocate the payment, I've sent proof of payment more than once and the feedback I receive is that the money will be allocated

I told the agent that the refence was my id number and I was told they will trace and allocate the payment so I'm confused why am I getting the same call again and always explaining that I've paid. If this issue is not resolved I will have to take further steps.