BAD SERVICE - MIX TELEMATICS

I had a Beame tracking unit installed in oneof my cars. fter about a year and a half I sold the car on 3 January 2019. I advised Mix on the same day via email. Since then I have been billed monthly via debit order which I had reversed by the bank. I received a SMS fom SCS saying that I have been handed over. I responded with my reason and the fact that I have advised Mix. SCS suggested I phone or email Mix at customer care. I sent proof via email of sale and a copy of cancellation request dated 3 January. I received a query number ********** 8 on 29 May 2019 via email. I also tried to contact Mix multiple times via telephone on 3 June as well as 4 June. No answer from Customer care or account queries. Otherwise I have been cut off. The English-only people manning the stations may have been absent?? I have today again received a SMS to say what consequences I will face if I do not pay. What else can I do??????????????