Contract Dispute and account debited.

I cancelled my contract with MTN about 2 months ago(which they confirmed with me that it was cancelled and transferred to prepaid).2 months down the line they decided to debit my account again without my consent or having a contract with them. I went to a store to have this resolved and they told me to dial 135 customer service.I tried the number but 1 hour later I was still waiting for a consultant to pick up or answer the call.It seems like there is no way of getting hold of them.Even after raising an incident on there website the response is that they will get back to you within 8 business hours or alternatively contact the call centre between Mon - Fri 08H00 - 17H00.