How react to them ripping me off

So basically I was paying debts to debt collectors ( xxx ) since 2020. The debts were filled by October 2023. I have not realised that and the automatic transfer was going on. I realised that lately and stopped the payments. I called them and seems the organisation is taken by xxxx corp.

I want them to refund me. But they keep delay my request and ignoring my emails.

How can I complain? Which department I should go to?