Cell C won't let me cancel contract I didn't want

I was on a TopUp contract: meaning when I run out of airtime it moves to prepaid<br> <br> CellC phoned in Sept, offering renewal & asked what phone I would like. I said I don't need a new phone, so they offered me a data ****le instead, BUT didn't say that it would change my number to a DATA ONLY plan.<br> <br> I connected my laptop to my phone for internet, and it was turtle slow. Decided to check airtime, and it said \Current usage: R 680"... They'd moved me to a data plan with no cap"