Unnecessary delay

I had been unable to pay my account with home choice due to unforeseen circumstances. But when I could I contacted their debt collectors at the time and made arrangements to pay. I set up a debit order that was meant to go off for four months until the account was settled. After two months of payments, they changed their debt collectors to DMC and handed over my account without updating the payments that I had made. For about a month I've been getting calls from these untrained agents all telling me that I have not made any payments since 2016. I've sent DMC proof of payment but each time I call in, these untrained agents can never give me answers on a way forward. I have asked to speak to a manager or to be called back but non of this happens. So I'm sitting here with a bad credit record for something that would have been settled by now if it weren't for the unprofessionalism of home choice and their debt collectors.