Contract debit ordet overcharged

I was offered a tablet contract on 26 October 2015,agreement was that I will pay R120.00 a month which includes 500 megabytes free for 24 month's. 27 November I was charged R151.46 which was more than agreed contract amount. I then phoned Cell C to enquire, was told that I was on open line which I was not aware of. I requested open line to be closed. I was promised it is closed & am converted to topup, will be charged R120 going forward. On 28 Dec i was charged correctly a R120.00. To my surprise end of February I was charged R896.00. When i phone to equiry i was told it open line data which i requested it to be closed. I have asked how come was i charged on open line as i had inclusive data on the day i used internet and had requested open line to be closed before. The issue has been escaletd since 29 Feb but no one is phoning me back with clarity on how come was I charged this amount. Cell c has ****** me R890 that i did not budget for & phoning call centre does not help as u get transfered from pilar to post then they drop the call if they have an answer. Reference number of the case is ***. Cell no. ***. Please i need help to get my money back from Cell.