Attempting to collect debt for a legitimately closed account

A terrible telecommunications company of note if not ****my or *********. What a tormenting matter it is. I have paid up my account in 2016, received a paid up letter from the Debt Collectors whom were appointed to handle my account. Years after, Ismart resurfaced telling me to pay the outstanding balance amounting to R3000+. I sent them the paid up letter and they disappeared for some time. Then, they came back with Debt Collectors, I checked my account status with the Debt Collectors I paid my account to and they comfirmed a 0 (zero) balance and sent the same paid up letter to their Debt Collectors - disappeared again. Lately, they resurfaced with another Debt Collection Company, I am now feeling like this company is a **** or **********. I even sent an email to Ismart informing them of my intentions to pursue legal path on this matter, they haven't reverted back to me. This operate purely like ****.