Poor record-keeping and communication, wrongful debt collection

I lost my job two years ago and fell into debt. I got back on my feet and settled all my accounts in full, obtained legal and lawful settlement letters, and all accounts were closed subsequently. This was done during MARCH/APRIL 2024. Imagine my surprise when, out of the blue and months later, I suddenly get SMS's and calls from this debt collecting company stating I owe money on the very same accounts which I have settlement letters for? My oh my, and judging by all the reviews before me, I'm not the only one with this problem. To make matters worse, and which, I presume, is the reason why this so-called reputable company has so many bad reviews, communicating with anyone is a feat in itself. After stating to the call center agent that I have settled this debt and the accounts have been closed MONTHS ago, and sending emails to their official email address, with my settlement letters attached, I get a reply with no introduction, no hello, and an invoice attached. They do not even have the decency to give a detailed description of which account it is they are claiming for:) I've responded again. I've reported them to the ombudsman for financial services. My question is, how is this allowed to happen? What went wrong? Months after the fact you decide to join the debt collecting party. And don't tell me that the company recently handed me over to you. The company has the records that I settled everything in full and that the accounts are closed. I am in possession of the legal and lawful settlement letters. The same ones I sent to you which you promptly ignored and rep**** to with an invoice. It's not my fault that you made your first telephone calls to me months after the accounts were already settled. It's not my fault that you lack due diligence and record-keeping. Now, I ask you once again, respectfully, to remove my name and details, especially my telephone number, from your records.