Nimble Group harassment for old settled contract

Back in 2014 or 2015, I had settled a contract managed by Altech Autopage in full by paying cash, the amount of over R3500. I recall being extremely upset when I was forced to pay R109 for a prepaid sim card so that I could retain my cellular number via porting to CellC. All was fine after this, my account was changed from contract to prepaid (something that CANNOT be done while monies are outstanding on a cellular contract). I then start getting emails and SMS's from this Nimble Group this year who requested me to pay R1086.41 for the Autopage contract. By this time, I have no proof of my payment, not all people keep receipts dating back 3 to 4 years. I have sent numerous emails for them to please contact the Autopage correspondents they bought the debt from but no luck, same auto emails requesting this money. No matter how I explained about cellular accounts that cannot be migrated to prepaid if the contract has not been settled but no luck with these guys. My last email to them has been for them to produce a letter withan Autopage company letterhead where AutoPage states that I owe this money and also that I AM able to have a prepaid number while I still owe money on the contract. Only response from Nimble was another auto email with their banking details and amount I owe them. I really need someone at Nimble Group to actually get a human behind their emails and actually read and also investigate the account because this is now harassment. This also better not affect my credit status at Transunion ITC. I would appreciate Nimble Group responding to this review.