Our telco provided POTS backup lines to our PRI. If the pri was unreachable or busy, any call to any DID would be routed to a bank of POTS lines. We also had the ability to do remote call forwarding to the main hunt number of those backup lines to forward to an offsite location if necessary.
Our incoming toll free numbers used verizon's enterprise routing. From a website i could swap routes and have any of them set to redirect to an predesignated route, a quick route or announce a message. Even in the event of a telco loss, i could route to vonage, roadrunner or some other telco.
Also kept a small key system in storage that could be quickly deployed to provide some phone functionality in the event of a major disaster.
The auditors always gave me a check-mark.