Difficulty cancelling contract

I am selling a vehicle that is monitored by Tracker and so need to discontinue the service. I have phoned 4 times to do this. The retentions and cancellations option at the call centre has never been answered, despite my having hung on for up to 20 minutes at a time. There is simply repetitive music. No one even indicates that they are still aware of your call. I have also sent 2 emails, neither of which have been rep**** to. I am sure that if I was offering new business, my communications would have been dealt with promptly.