In previous chapters, we discussed trunks and dial plans. How do you backup those if one goes down? You have to set up several trunks and then, in your dial plan, set the other trunks as 2nd or 3rd options:
Just make sure your Strip Digits and Prepend rules apply correctly to those lines.
That covers outbound backup, but what about inbound? This is the tricky part. You will need to talk to your PSTN or ITSP vendor and see what options they offer. Some have roll-over options, where the call will go to another number in their system.
Others have a "disaster recovery" option, where they will forward the incoming call to any phone number you want. Take advantage of these options as your lines will go down someday.