Suppose to be a top up and not an open line

In April 2014 I took out a contract for a USB modem with 1 GB - total amount R82.90 (excl vat) per month. I requested a top up and not a open line because it is for my daughter to use and she is still in school. Since April 2014 up to July 2015 my bills were 100% correct. All of a sudden, my account last month was R2 935.57. I've made an enquiry only to be told that it is not a top up but an open line. I requested a copy of the agreement and a copy of the voice recording as it was done by phone. Autopage could not trace any recordings or signed agreement and insist that I have to pay the account. They can't mygrate the account unless it is paid up to date. Please help