Cancelation

I have sent an email to MIX Telematics regarding the cancelation of my device as the car is sold. They have never responded to me with any update. Every time you call the accounts department you cannot get through. I spoke to Sarah today at the call centre and asked to speak to someone senior and she transferred me back to the call centre (10min holding on the phone). The service is absolutely disgusting and I have been called 3 weeks ago to advise me that the battery is dead in my unit for some time and is no longer working and I did advise them I will be cancelling and they are now still charging me. Worst service I have ever received.