Account not up to date and the consultants that call me are rude

In February I uncounted financial problems and resulted in not paying my car instalment , I was called by MFC and I did make a payment arrangement that I will pay the instalment over two months plus my original instalment . In March and April I did exactly that even thou they were now trying to double change my account plus the arrangement amount , My account was a mess so I resulted in reversing the amount that were not correct and continued paying the instalment plus with the two arrears that have been corrected .Now every month I'm being billed for an extra amount that I have no idea where it comes from , Unfortunately where I work cell phones are not allowed and by the time I knock off I get missed calls from them ,when I try to call them back they have closed for the day . One agent recently called after hours and I was able to answer but instead he was rude and sent me a 5 years statement as if that's even possible . Then I got another voicemail from a lady at MFC whom was already threatening to take my car as I cant afford it ,I mean what the hell is that ? been paying it for the past 5 years and now suddenly I cant ? just because I don't agree with the amount .I'm not sure where should I go to now as I honestly feel that I have paid the amount and I need a proper investigation done ,if the mistake is on my side then I'm happy to settle the balance but if not then please rectify this asap !!!