Having to pay for billing error

I am exasperated after dealing with Cell C. This is the second time I am writing a review here because I have still not had my issue adequately resolved.

I was being overcharged by Cell C on my Fibre connection, as they charged my R1399 a month for a service that is actually only R1199 a month. When I queried this, it took a number of calls and complaints for them to understand the issue and to finally fix it after admitting the mistake. However, when they fixed the error, the only corrected it prospectively, meaning that all the incorrect amounts I was billed over the past couple of months were not reversed.

So now my monthly bill is correct going forwards, but they still expect me to repay the excess amounts they charged me even though they admitted this was a billing error. And to top it off, the debt collection department is now threatening to blacklist me for this.

I don't know what more to do anymore, this is disappointing and unjust to say the least.