With the appropriate equipment, Avaya Aura CM sites can “survive” an outage. An outage, in this context, means that the site lost IP (or TDM shudder) connectivity to both its primary controller and its enterprise survivable server (if there is one). The necessary equipment includes (generally) a media gateway with a local survivable processor, and some form of trunking (if you want calls to be able to leave the gateway).
If you read Vectoring - “Intercepts” and forwarding callers to other numbers, you probably realized that following the steps in that post to silently forward callers to a different destination using VDN variables can be made much less complex than what that required. Indeed it is: there’s only one vector step needed: route-to.