Being billed after the account has been settled

I have been receiving calls daily from Norman Bissett for outstanding amounts which is being incurred after the settlement for calls being made to a paid account. The settlement arrangement was dated 26 July 2017 and payment made 27 July 2017. I keep being advised that this will be updated yet the calls don't stop. My Nashua Mobile Account was settled in 2006 and because I could not find the proof I had to pay the R286 just cause I do not want this on my account. Please can you update your details and reverse any cost which is being added to my name. I also want a letter confirming this is paid in full.