Vumatel - outstanding installation fee

More than 18 months after my fibre installation, Vumatel collections keep harassing me regarding an outstanding installation fee. However, my fibre deal with my ISP included installation and I have received an invoice from them showing that this is paid for. It is worth noting that I have not received an invoice from Vumatel for this "outstanding installation fee". I have told Vumatel collections this on numerous occasions but they do not take note of anything I say. This is beyond frustrating.

If anyone has had the same problem and succeeded in sorting this out, please let me know how to proceed with this issue. I will probably never be using Vumatel again after this.

Thanks.