Account wrongfully handed over

I had a cheque account that had an overdraft facility with standard bank. Due to bad service and branches around me closing down, I decided to switch to another bank. After switching I made several calls for the accounts to be closed and I was never offered any help while there was bank charges on the account I was not using. To my surprise 2 years later I get messages and calls from a debt collecting company telling me that Standard bank has handed me over and this is after I made a payment of R35 168 in 2019. When I try to get the mess sorted I'm still being sent from one department to the other and I'm using cellphone airtime to phone them which is costly. Please sort out this thing because it has an adverse impact on my credit profile.