Mtn did not cancel my contract.

For number ***, I was eligible for an upgrade in 2015/08/06, when I called to cancel on 2015/11/19, I was told to call on the 6th December 2015 or the 7th in order for me cancel totally, ref number 138 656 760. I ten called on the 7th December 2015, to go through the process of cancelling, I was told the contract was cancelled, and given a reference of 139 635 351. I was happy to have cancelled and I never bothered to follow up, only to find out on the 1st March 2016 that MTN is still billing me for a number that I do not even use anymore. When I called, I was 2016/03/01, the call was prematurely cut off twice, until a lady called Glen assisted. She could see that i did call on the 7th December 2015, but there was no note on the system to actually say I am cancelling the contract. So MTN has been billing me for December 2015, January 2016 and February 2016, for someone not doing their job correctly. I want to be reimbursed for the money they took off my account, who is liable for this. Every time you call MTN they blame the system, but this was not a system error, it was someone who didnt follow instruction to cancel my and ended doing what they wanted. I need answers.