Incompetence ruining my credit score

I am absolutely livid with Vodacom. I cancelled my contract on the 22nd of January 2020 which was 1 month notice on account I***. Every month I received the statement saying I’m in arrears therefore I would call and advise that I cancelled my contract they would confirm this and tell me it will be removed it was an error. I am so annoyed with the calls I’ve been receiving daily advising me of the arrears. Even when I call their customer care and cancellations somebody is “supposed” call me to rectify yet all I’m getting are calls of demand. They even have the nerve to say it will affect my credit score. Their terms and conditions state the cancellation must be done a month prior to expiry. I demand that they rectify this as I refuse to have a tarnished record due to Vodacom’s incompetence!