Customer for 12 years and agent hangs up on me

As a result of Cell C *****ing up their billing system a number of years ago i moved across to paying by EFT. so every month i do a transfer and every month the amount showing on my statement as outstanding and the actual amount differ. This morning i get an sms saying my account is in arrears and will be disconnected today. I check and i had made a thousand rand payment last week so i look at the online invoice and it states the amount should have been 1251. So i make a payment of R255 and then call 140. Firstly the agent tells me the line is bad i must talk louder, i stated well seeing i am using the Cell C network they should maybe sort out the quality issues. I then ask what is outstanding and get told it is a whole R36 ... they they have sent me a threatening sms over R36 .. on a 12 year old account where if i run the maths i have paid in the region of R200 000.00 to cell c over the years. I then ask the agent to confirm the email address for proof of payment for the R255 i had just paid at which point he hangs up and send em to the survey prompt. At this point after the terrible service Cell C is very good at delivering I am at the point where I will pay another Network R10k.