Unexplained debit order deduction

I took out a R99 pm 2GB data contract with Nashua Mobile (via CELL-C) in November 2012. (That's 32 months ago). When it reaches 2GB, the sim card soft locks. For 32 months, R99 has been deducted. Then this month, R5 513 is deducted. I contacted the call centre on 01/07/2015 and spoke to Tsepiso. She in turn just kept on saying that it is data charges I have incurred and refused a mere 15 times to put me through to a manager. Yes , I asked her 15 times. She just kept saying that she is not going to transfer me because she has given me the answer to my question. Eventually after Tsepiso put the phone down in my ear, I called again and spoke to a manager, Thato, who said that I should reverse the debit order and that she will log a dispute. Eventually she emailed me and said that I am liable for the data usage. How can the written contract that I have in place with Nashua Mobile be amended by AutoPage without any documents/consent from AutoPage to back it up? I have all my Nashua Mobile documents showing I signed up for a R99 pm 2GB contract. How can data usage be incurred WHILST A SIM CARD IS SOFT LOCKED ONCE IT REACHES 2GB?