I've got a problem with Altech Netstar

I've got a problem with Altech Netstar. I sold my car in October 2015, before I sold it, I called them, I was helped by the Dealership whom I sold my car to make a call, since the automated system was making them unreachable at that time, as I was using airtime. I informed that that I was selling the car, I even told them when the car was no longer in my possession. In 2016 they sent me the invoices with large amounts which they said I was owing them. They handed me over to their attorneys, when I explained to the lawyers my side of the story, they stopped calling me. In 2018 they called again, I told them the same story. Even now in 2022 they still claim that I owe them. They didn't disconnect /suspend connection, is that my fault? How am I supposed to pay for somebody else when the car is not even in my possession?