Robbing clients

We had cellphone contract with mtn which was supposed to end on may 2020 it didn't stop so we stopped it and cancelled the contract while we were waiting for paidup letter doing follow up on January we were told we owe them R2165 we asked what is it for no answer and we were handed over to the debt collector's I asked how because we were waiting for a paid up letter where is this money comes from no answer called 135 many times from January till now no answer then received call from debt collector's saying we owe mtn 11 thousand on September 2020 we were handed over to them we ask them for a statement because all we have been waiting for is paid up letter so we can continue with our lives last week received email from mtn it shows right on may we finished paying it was 0 then boom on January R2165 we want to know what can we do because at mtn we dont get help how can we pay money that we already paid