Denying payment

I have two Cell C contracts. After being retrenched August 2019 I managed to pay the account via debit order until December/January when I notified Cell C that the Nedbank account is closed. They were given new account details. However they carried on debiting the wrong account, causing me to have to pay EFT return penalities of 70.00 per debit order. I contacted Cell C again, and again requested that they changed my banking details and also enquired about the penalties which I believe I am not liable for. I made EFT payments on my accounts in January, on the same day. However, they "can not" trace the payment for my personal phone. I went to Capitec to get proof of the payment and mailed it to Cell C. The lady I spoke to said they will sort out my query. It still has not been done. My account was debited again in April for the amount I paid previously which Cell C claimed they can not find in the system. I spoke to another person at Cell C and she said they will investigate. I received an sms indicating they will contact me. Late I received a sms to say I must indicate if I am happy with the reply and correspondence, but I did not even hear anything from Cell C regarding the query. I am utterly frustrated with Cell C and I am contemplating cancelling my contract with them.