Breach of Contract

My mom had a contract with cell C years back and she paid it and finished the contract and she was subscribed for data and early this year she was contacted by Cell C to pay the amount of R719.03 which she paid end of March 2015 and she even faxed the slips to Cell C ,and was advised by the consultant that it is the last payment that she will be making and was given a Nedbank account number to put the money in. Now early this month of May 2015 she received an sms that she needs to pay R1000.00 couple of rands and she was given an FNB account details. Kindly please assist my mother as she is getting thousands calls from Cell C and they all tell her different stories and she is getting frustrated by this calls. Can Cell C cancel my mother's name on their roll as she is older and this consultant trick her into signing up for things she doesn't even understand. my mother is cripple and she cant be running from Cell C outlets time to time she gets such smses and Cell C is even threatening her with the lawyers.