Account Paid in full and no response from this company

On the 4th May 2023 I received a settlement offer from this company for one of my accounts that was handed over. I paid the amount and send them the proof of payment. On request for my paid up letter I am just not getting any feedback. The account that was paid up in full, is still phoning me for payment and I am not getting this resolved as I don't have a the paid up letter. When will this be resolved and when will I be able to get a statement showing I don't owe anything?