Useless service/theft

I am doing my mother's financial business duo to her old age and the fact that she can't remember well. Before she moved to an old age home, we went to MTN in Krugersdorp (President square) to cancel her contract. The people at MTN helped me and phoned to MTN to cancel the contract. They also confirmed that my mother was sitting (wheelchair) in front of them and that she wanted to cancel the contract. At that stage we gave them a copy of her ID. They told the guy at MTN that they will phone her later that afternoon, which did not happened . This happened during December 2019. Since then I tried five(5) more times to cancel the contract, but just get a cold shoulder from MTN. In the meantime they withdraw (steel/*******ion?) more money from her bank account without any reason/notice. Her monthly payment went from R 379.55 to R 770.00. Their service is useless. I also get the impression that they steel from old age customers that can't help themself.