vumatel accounts unsubstantiated harassment

I was issued with a vumatel installation invoice which i promptly paid and sent proof of payment to vumatel. Since August 2018 i have been harassed and accused of non payment despite re sending the proof of payment. Vumatel accounts department told me that everything would be sorted out and yet again this morning i receive an sms "reminding" me of my outstanding payment to vumatel.

i think it is absolutely ridiculous that this situation is still ongoing and that no attempt has been made to rectify this from vumatel side.