I have been experimenting with FreePBX 14 with Asterisk 15. I have two instances of this on KVM instances. If you dump system “A” and restore to system “B”, the extensions are missing. They are present in the dump files, but never get loaded to /etc/asterisk.
This could be a show-stopper for migration to Asterisk 15
FWIW, it works fine on FreePBX14 and Asterisk 13. Haven’t tried Asterisk 14.