Cartrack phone call harassment

I am using a company phone, previous employer is a client of cartrack. Since he is no longer at the company he is not using the number anymore. I keep on getting phone calls from cartrack for his outstanding accounts. Even after blocking numerous numbers from cartrack I keep on being harassed with phone calls looking for the previous user of my number. Asking the call centre to remove my number is either falling on deaf ears or they are too *********** to either change it or add a note that the person no longer has thos number. Speaking to the managers is pointless as you are put on hold indefinitely. This has to be sorted