Bad customer service

I had a 24 month cell phone contract with MTN.Close to the expiration date of my contract early May, I phoned in to tell them to switch my contract to pre-paid.I was then told by a lady named Phatu I will receive a reference number.I never received it two days later where I phoned again and then was told by a lady named Liesel she will send a urgent email to attend to the matter.I also **** it very clear to Liesel that they must stop sending me airtime cos i don't want it cos I don't want it to be debited off my bank account.She then told me she will change my details to cash.My details was changed but I still got airtime and they expect me to pay for it.Just because MTN was on strike and people clearly wasn't doing their jobs, they expect me to pay my account which is now in arrears.And I never even received a statement from them since May.