Did you contact a fairphoneangel to test a working spare part? Are you still under warranty? In which case you should contact Fairphone support because it could be a factory defect.
But before this, you should indeed try @teezeh’s tips.
As you indeed said, there is this thread about reboots during calls.
I didn’t read the thread, but seeing your problem, you could suppress the contact with which you have problems and see if your phone still reboots. If it doesn’t, you can also try to add the contact again and see if it still works.