Phone and Broadband Nightmare

I changed my package with phone company, the new provider did everything, so far so good. My Direct Debit was taken out 30/11/2018 for usage up to end of contract date 21/12/2018. The changeover went without a hitch.I received 2 emails from old company at christmas telling me I owed money for the following month, £12 more than my bill used to be, up til 17/01/2019 !!I rang up and said I had left they had had notice and my contract ended. I was told if I didn't pay bill it would go to a debt collection agency, as in another 4/5 weeks I could have put a claim in to see if iwas entitled to anything back. I raised a complaint but they are not happy and I feel wrung out. Another friend is having the same problem.its a lot f money if everyone has to pay an extra month when they leave and there seems little you can do about it. I will have to wait to see what the outcome is.