Really bad customer service

I asked for a settlement amount, which was then paid. Proof was sent to them. They then still kept harrassing me for the payment. I asked for a settlement letter, which after MANY calls from me, was only provided on 18/01/2021. I am STILL being harrassed, and even letter sent to me, stating that I'm in default. When phoning them, one can't seem to speak to anyone that can help. And asking to speak to a manager, is completely impossible. I keep being fobbed off, and no manager have contacted me at all.