Insured car destroyed, insurance company not picking up the phone

The tracker puts a marker on the map, similar to the icon used for the Impound in offline player.

I believe the marker is called "Personal Vehicle" on the legend, so you should be able to find it fairly quickly.

Alternatively, if the marker isn't visible on the map, use your phone to call the insurance company and request a replacement.

You can't sell other players' vehicle(s). And you can set your personel vehicle to lock to everyone, yourself, friends, crew, crew + friends, and no-one. To change your vehicle's lock setting, go to: Interactive Menu >> Scroll down to Vehicle Access >> Select your setting.