Closed account in arrear and handed over but payment was made

When I sold my house in 2017 I gave a months notice to ADT however the last premium was never allocated to my account leaving an incorrect outstanding amount of R418.17. I stopped receiving statements but then suddenly received an SMS saying that the debt had been ceded to another company. I sent proof of the 2017 payment to the accounts payment but have never had confirmation that this has been resolved. I have never had a problem with their reponse to callouts but their accounting system is shocking, payments don't get allocated correctly so you're continuously in arrears.