Poor service from cell C

My mother took a contract for 5 years with cell c. She is over the age of 60 years. The contract was ending on September this year. Cell c kept on calling her to renew her contract but she said No, she is not renewing the contract. Now they say, she must pay R1400 because she did not cancel the contract and they increased that amount from R274 that she was paying monthly to R710 without her knowledge. I find this unfair because they increased this without her knowing and agreeing to that amount. Now she wants to cancel but they are refusing. She says she can pay the R274 for October and November but they are refusing and they do not want to help us. Their consultant are rude and no one is giving us a solution. They are threatening her. They say, they will take her to the lawyers.