[asterisk-users] Channel failover

[Jack Bates]

How do you configure Asterisk to dial, in order, each channel from a
group of channels until it either finds an available channel, or runs
out of channels?

We recently got VoIP, so when we make a call, Asterisk should first try
to make the call with VoIP, but in case either our VoIP or our internet
service are down, Asterisk should then try to make the call with our old
school analog phone line

I searched for configuration parameters to put channels in a group, and
then dial that group in order until an available channel is found - no
luck

I also searched the sample configurations for an example, likewise
without luck

Finally I expected that this topic would be covered in the O'Reilly
Asterisk book, http://asteriskdocs.org/

- but again, I found nothing on this topic specifically

How do you configure Asterisk to accomplish this?