Cartrack Unprofessional Account Office

I had a month-to-month service with Cartrack and enquired as to how I can end the service. The agent explained to me that I just inform them and pay all fees for services rendered, it can be cancelled. This was done and I received a last invoice for R16-77 that was paid. Now they will harass me with phone calls claiming that I still owe them R129-00 that is equal to one month service. Upon enquiry I was informed it was a contract cancellation fee. I never had a contract with them. With every phone call I receive, I request explanation on whet the money is for, but all the agents cannot explain. I just owe them R129-00. If they can supply me with proof where I signed that I will pay a cancellation fee, then I will pay, but they cannot.