inccorect contract deal

my Name is Makabongwe Gcwabe *** - Cell C upon expiry of my Contract contacted me to renew my contract and close the then contract number and move the new contract ( as was before) to my prepaid number but after a while they contacted me to say I owed more than 500 while that is not the new contract terms I agreed to , I called them to tell them this was a mistake but they refuse to cancel the wrong contract saying I agreed to it when I ask for proof knowing full well I would have never agreed to the terms of the new contract because I am on debt review and can not afford to pay more than I did on my pervious contact , they couldn't provide that and also refuse to cancel the contract I was misled into entering into.