disgusting and poor customer service

my mother paid up her contract in march 2015, they even gave her a cancellation letter to sign during that process the sim card was discountinued, no data or airtime was loaded on that number in january 2016 she started getting calls from cell c collections telling her about the contract she told them it was paid up and cancelled they refered her to he main branch where she app**** for the contract, she went to maponya mall the consultant reassured her that there's nothing owing on her account, she must ignore calls from the call centre until this week she received another call telling her about a balance due, wen to maponya mall again she's now told she's owing, she provides them with all proof of payments an the cancellation request paper which she completed at the same branch maponya mall but they were no helpful at all besides telling her she owes cellc. i am very disgusted because non of you staff is willing to listening with understanding or try to help my mother who is a pensioner now because they were suppose to make copies of her documents and escalate the query cause my mother doesn't owe you guys nothing and that sim card you guys blocked it long ago. fix this nonsense