Harassment of customer for funds NOT owed

I gave notice to cancel my month-to-month contract on 27 March. I was told that the service would terminate on 23 April, that I do not owe any additional funds and that my account would be debited for the last time on 25 April. I then received a Whatsapp on 03 April that I owe a "returned debit" which does not reflect on my bank account. I have sent numerous emails and am still awaiting a follow up from Thando who said on 6 April one would be provided. In a previous email, Thando informed me it is for a "cancellation fee" which I was not informed of over the phone on 27 March, however I have since received a further 4 Whatsapps and a letter of demand for a "returned debit". I just got phone call (13 April) from Kentse who told me it's a "cancellation fee"; so which is it? Note I have not received an invoice for any cancellation fee. She then agreed with me that the service will terminate on 23 April and I will be debited the last time on 25 April and that I do not owe any other funds when that debit goes off. So what happened to the cancellation fee? I also told Kentse that Cartrack is harassing me for money I do not owe as the final debit will go off on 25 April as their employee told me on 27 March and that I do not owe any other funds. Kentse told me it is not harassment even though I have received 4 Whatsapps, 1 emailed letter of demand and 12 calls from various Cartrack numbers for the "returned debit"/ "cancellation fee". She said that it is automated and she is not responsible for it. Actually, the accounts department provides the information for the automated system so yes, the accounts department or a representative from Cartrack can stop it. Cartrack can't even get their story straight about whether or not I owe money. If this is what their administration is like, how are they able to find someone's car should it be ******?