Still invoiced after cancellation

We have had a contract for several years and phoned to cancel on the expiry of the contract as we no longer want or need the service. Although the customer service system registered the call made the instruction to cancel the contract was not actioned and debit orders continued to be processed through my bank account. Again we called and this time received confirmation sms that the agreement was terminated. Imagine my surprise when once again we receive a tax invoice for a period after the termination date. We phoned customer service again and spoke to Thando who asked me 4 so called security question and kept wanting more information. This man's attitude is one of hindrance rather than assistance and to add insult to injury the call was dropped. On inspection of the tax invoices we have been double invoiced for data for August which is when the contract was due to be terminated. I am just given the runaround when I want to cancel. Customer Service is atrocious.