My new Nexus 5X was a bit troublesome on pure Android as I was used to Cyanogenmod (nightly snapshots). So I had to do what a girl has to do: Flash that thing. Add Gapps and be happy.
Practically, the phone was rebooting every time a phone call was made or received. What I did miss was that if a device has Google Dialer installed, it requires to be set as the default Phone application instead of ’empty’. If not set, it will reboot your device.
To fix this, go to ‘Settings -> Apps -> “Gear Icon” upper right -> Default Apps -> Phone App’ to set Google Dialer as your default and you’re good to again.