Extreme poor customer service

I have been with Cell C for over 10 years, I recently got my message stating I am due for upgrade, I tried calling several times for someone to assist me as they declined my contract renewal. they also declined 2 years ago and override this. The reason I am being declined as Telkom shows a outstanding balance - however this is more than 5 years old and I also paid the account in full 7 months ago but cannot get anyone to remove it from my name. Every time I call Cell C they inform me they will call me back and no one ever does. I am starting to feel like their employees get paid to make customers lives difficult and stress them out. I just want someone to help me and this seems impossible