Yesterday my Freepbx’s DNS servers were unavailable which in turn caused the phone system to be unavailable for internal and external calls.
How can I prevent the phone from going down during a DNA outage?
In /etc/resolv.conf, add another non-freePBX DNS server listing. It will try in order the DNS servers until it finds one that works.
That still does not protect you from loss of internal calling when the internet goes down.
You should make sure you have caching nameserver setup. Not sure if Asterisk fixed this in more recent version but it won’t hurt to have this installed anyways.
http://www.powerpbx.org/content/freepbx-asterisk-and-linux-advanced-topics