Fail to respond to complaint on excess billing

I have a fixed contract with a hard cap at 300meg data per month. Last month I received a bill for R250+ more than my standard contract (and I generally use less than my contractual value so only pay the fixed amount). On analysis it turns out that the hard cap did not kick in and they started charging me for data used without any warning (its not like I dont have 141 that I can convert). Sending an email to the call centre was met with a response to answer security questions relatively soon after the query to which I responded with all but the pin as I do not have one (am a Nashuamobile transfer). As of 4/8/2015 I have yet to receive a response or a call to deal with the issue. In my note 11/8/2015 I explained that I required a response and that they are welcome to call me if they need to discuss the matter. To date nothing. I understand a colleague has the same issue on a hard cap and is also struggling to get assistance. My call number is [#***7]. This comes hot on the heels of them double billing my credit card last month because they had missed the previous month - given that this is a debit order it was their failure to initiate the previous month.