overcharge

have a contract with Cell C since last year and I have been billed correct the R999 unlimited calls, with 3 GB. Last month they debited R2799 from my account and I immediately called to ask the reason. The first response I got was that I have upgraded my account with a code I dialled from my phone, t, also I made an international call, I used data for the amount of R1799 and lastly I was told I have a limit of R2500 to my account which I am not aware of. they say from the time you take the contract the credit check you and they give you a limit you can afford and this was never told to me as I would have refused because the reason I took the contract was to reduce my expenditure on calls so what purpose would that have served if still I have credit of R2500. for the previous months If I had reached my limit on the data I would not be able to send or receive emails, what's up etc. and I would have to buy the data which shows on my previous statements. Only last month I learn I have this increased credit without my approval and I asked where in my contract does this appear but to no answer.