Very annoying calls even weekends and Sundays

Appalling business practice. So i sold my vehicle which was fitted with a cartrack month to month device. I then informed car track of this and they asked i provide the new owners details so they can arrange or get him as their client. Which i did. I have recently received a text saying i am listed as a DEFAULTER/BAD PAYER. This is unacceptable if they need money they should ask for it than try ***** it from us client. I WILL NEVER USE THIS SERVICE AGAIN, thinking i almost called them to install a new device for my new car. Fix this or am going on social media to share my grief with the world.