Contract cancellation not done after confirmation email was received

I send an email on 30 November 2020 to Cell C customer service to cancel my cellphone contract which end January 2021. I received a confirmation of the cancellation on 4 December 2020

Email from Customer Service : Thanks for contacting Cell C with your request.

We are sorry to hear that you wish to cancel your contract with us. We sincerely hope that you would reconsider your decision to cancel and consider future deals to further enjoy Cell C benefits.

Your contract will be cancelled on the 19/01/2021 as we require thirty days’ notice and that is the end date of your contract. The last bill will be in January 2021

Also note that the unused benefits will be forfeited however, you can contact us to change the date to allow yourself time to use the benefits as you will get full benefits for the month and be billed for in full.

Kind regards Retentions Department

Today, 1st February I received again monthly Cell C benefits. With my data being on some data was unknowingly used due to applications running in the background. I phoned Cell C, who informed me that on their system my contract is still active and that they will cancel it, but I need to send the cancellation email and need to pay for the benefits used. Their service is pathetic for it's not my fault. Now I need to pay for their mistake and staff who is too lazy to do their work properly. This is unacceptable. I will cancel my other 2 contracts as well when those contracts ends. I am done doing business with Cell C.