Billing Usage

on 16 March 2015, i received an sms saying that my unbilled usage is R1219.23. I was under the impression that i was on a top up contract. when i queried with the customer service, they advised me that i was on an open contract. I have two contracts of the same nature, why would i ensure that the one is on top up and the other isnt. Customer service said that i need to call the branch where i renewed my contract, which i did, on 18 March. They then changed my plan to a top up, and said they would investigate my usage. My line was then suspended on 19 or 20th March. Only today, did i get some feedback as to why i was billed. I know that I only got 100mb, and therefore was buying data everymonth. Now i get told, that i used the bought data and got billed for data, which doesnt make sense, and i am furious about this. What ever happended to TCF? no one explained this to me, I was not aware of this. i feel that i need to be compensated for the data i bought, now my bill is R1829??? My line is still suspended and i have no idea as to why, because my bank account gets debited on 01 of every month. This is not the first issue i have with Altech and will never renew my contract again.