Appalling Billing and Customer Service

Looking at the latest reviews, I'm not the only one who has received an SMS, to pay an unsettled account by the 20 November. This after cancelling my contract on 14 October 2021. I moved to a different ISP since the cancellation, and in February 2022, I received an invoice for services for the month of February 2022. I queried with a customer agent, sent emails confirming the cancellation and was told that a credit note was in process with the billing department. I have not received any correspondence since 18 February 2022. 16 November 2023, I received the SMS for final notice for payment of the same invoice I queried in February 2022. Failure to make payment will result in the account being handed over to an external collection agency. I sent an email the same day of receiving the SMS to accounts, with a reply stating that *Thank you for reaching out to us. We are committed to resolving your issue as soon as possible. One of our Customer Care Consultants will be in touch shortly to resolve your issue.* I have not received any communication telephonically or via email. This is unacceptable and surely something is not right with your system. How is it possible to receive an invoice 3 months after cancelling, invoicing for a month that I was using a different ISP? Surely this is against consumer rights?