Mix Telematics has bad business ethics

Asked for the service, within the first weeks I was notified that they cannot track me (faulty unit) I must go back to the contractor Somerset Car radio and security. After very unprofessional service they showed me away and said they will call me. Same with Mix Telematics, they said they would call, but never did. Thus... I never actually had the service, they knew and kept billing me. I eventually was told that I had to cancel before anyone would react... Which was 100 phonecalls later. Now I was handed over and gets 1 - 3 phone calls a day from the debt collector. I cannot make any contact with Mix Telematics for help from their side, they also don't respond to my emails. They now threaten to black list me for less that R700. This is not acceptable. Pls help!