"Final Ntoice" on a contract that is and nver was in arears

I was a Altech Auto Page customer before they closed down, when they closed down +/- 2 or 3 years ago the account was transferred over directly to MTN. At the time this happened I received a SMS from them stating that I should pay my monthly subscription directly to MTN going forward and they also provided me with the banking details I needed to use going forward. I therefore started paying it directly to MTN, and I had no areas on this account. We never got any statements or messages from Altech Auto Page after that, certainly nothing stating that we still ow them any money and we kept on paying MTN each month. Then suddenly at the end of 2016 (almost a year later) I suddenly started getting messages and threatening calls from Nimble Group saying that I have been handed over for none payment on my Altech Auto Page account. I keep telling them this account is not in areas and was transferred over to MTN and the matter was then eventually escalated with in Nimble Group, after which I then did not get any feedback or heard from them for several months. The suddenly it started all over again, I got several SMS's and constant calls from Nimble Group threatening to hand me over and blacklist me. Over the last three hears or so this cycle keeps taking place. I have asked on several occasion that they proved me with some proof from Altech Auto Page that I was in areas and for which month or period they are saying I did not pay but they are unable to provide me with anything, just an amount and the phone number of the Altech Auto Page account. Now I'm getting messages and calls again regarding a final notice. I even did an upgrade on this contract with MTN when it reached it's 24month expiry and go a new handset. Why would MTN give me an upgrade on this account if it was in arrears?