Unnecessary demands

I had to cancel my contract with Atlas in December since I was moving to a new house. My account was up to day,I did not have any outstanding amount since they were deducting straight from my account. To my surprise after that I received endless statement from them with outstanding payment and today I received a WhatsApp message from their lawyers,demanding a payment within 20 days or I'll be registered on credit bearou I'm so disappointed with them because I don't even know what the money they claim I owe them is it for.