Phone soft reboots when making or receiving a call [Nexus 6][CM13]
The issue is the current build of gapps - quick workaround is to manually set the Phone/Dialer app as the default Phone App in Settings -> Apps ->  -> Default Apps -> Phone App.

Source: https://forum.cyanogenmod.org/topic/124381-dialer-crashes/?do=findComment&amp;comment=590388