GTA V: Mors Mutual not answering? Here is your solution.
I sorta found a way to get a car in this situation back (One that is listed by the mechanic as destroyed but Mors mutual wont replace).
So what you do is get another insured personal vehicle and destroy it. Then you phone mors mutual and spam A (or I assume X on PS3) and hopefully this should return your lost car. And you will then be able to phone mors again and get your other car back too.
I think you then have to reinsure the previously lost car too, or you may have to repeat the process... I lost my car again before I could (A modder with a Uzi that shot grenadea got me...) and mors wouldnt answer again, I didnt try the trick again as I was about to log out.
Also note, this may be dependant on the car you lost appearing in the mechanic list higher than the one you choose to destroy for the trick, so it will be top of the list (or should be) top of the list for Mors too... Though Im not sure if thats necessary or not. Anywho, hope this works for others too.