Tracker Billing and customer service

Hi I sold my car and was billed for one extra month after cancelling the contract. I have been in contact with tracker since august and to date have send email and made various call reaching out to them. I have logged queries online and to date have not had any feedback. I was offered a voucher for 3month and even that has not been app**** to my account. They do not respond to emails or any online queries. I have to phone and waste 10-15 min for the person assisting to read the notes first and thereafter to be told that i credit controller needs to phone me back. I am seriously thinking of cancelling all cars registered with them as i have 3 that is covered by tracker Very disappointed!!!