Not disclosing all amounts on contract. From R150 to R500 without your knowledge

I am so disappointed in Telkom's behavior. I had a 2-year contract with them for a data deal for R150/month, at the end of the 24 months, their sales people called me for an upgrade to a slightly different deal, with more data at R149/month. I agreed and I specifically asked on the phone "Is this all I will be paying, and the lady says yes". I view my balance as I usually do, my bill looks normal online until the invoice comes. Apparently, they put a R500 spend limit on my invoice, but one which was not disclosed to me nor was it visible on my bill online. I called them the first time to clarify what is going bad and not in so many words, I was told "Too bad, you couldn’t smell that there was this sneaky amount you have to pay". They removed the spend limit but they told me I had to pay an extra R1000 for the 2 months. This was a service I did not agree to when making the upgrade and they had no right to add on their extra costs without consulting me. I went further to call their accounts department and asked them to remove their ridiculous cost. I was told it was the "Sales" department's fault for not telling me. I don't care who's fault it was, the fact is I am getting the service from TELKOM. The guy also told me there is nothing they can do about it, if I don't pay they will blacklist me whether it is fair or not, he even went on further to tell me "It doesn't matter who you call, there is noooooo one who will help you", this is a quote. I asked to speak to the manager, he apparently was unavailable, he promised to get back to me but it has been three weeks and I still get their ridiculous invoice and although I have continued to pay my agreed fee of R149/month they still cut my service. This is beyond unacceptable not to mention ********* and simply spitting in my face with a 'Too bad, so sad, pay up whether this was part of the contract or not'. In this day an age i canooooot believe that a company would treat a customer like that.