Charged for services not required/received

I've been with Cell C for almost 10 years, just upgraded my contract again in August/September. November bill came and I got billed for some data bundle I never requested or used as I am almost permanently on Wifi. I escalated this to accounts department as it doubled my normal bill and I can't and will not pay for that. So I paid what I correctly owed, and needless to say, Cell C disconnected my sim, and has not even rectified THEIR mistake. This is now 11 days later, and after a phonecall to their accounts yesterday, I was promised someone would still phone me yesterday, but I knew that my complaint is just falling on deaf ears - still waiting to be contacted!!! I'm so sick of service providers and suppliers demanding you pay them and if you default with one day they cut you off, but for them to sort out their mistakes they don't care if it takes years. Terrible customer service. I am done defending Cell C!