Prolonging a cancellation of the contract

I have a month to month data contract with Cell C. i have been battling with signal for a couple of months. I decided to cancel and sent them an email to as a notice to cancel. 10 days later i sent a follow up email, no reply. 3 weeks later i had not heard from them i called in to confirm if they had received my cancelation notice and that my last debit order was beginning November. Customer Care did a security check then told me they have to transfer me to the retention department. Retention department did a security check and then told me someone else needs to call me. they cant accept a cancellation via email they must call me . On Sunday i get a call from Cell C retention department, they do a security check and tell me i have failed the security i need to call customer service. i asked him how can i give the same information on 3 calls, previous 2 call security check is fine but on the third call it's not . Exactly the same information. He fails to answer and tells me he is not obliged to ask me more questions. so i asked to speak to his supervisor and i was on hold for 20 min thereafter i put down the line. All I want is a cancellation of this contract . Cell C, please stop prolonging this. As previously mentioned, i do not want another debit on this account as i cant even use it.