Failover or Redundant routing is a network arrangement with several links and paths between the person who places a call and the call recipient.

Should at any time a path or link in the network, the other links will automatically route the incoming call to a predetermined number, device or location.

Virtual phone numbers operated through a VoIP service provider include number redundancy management tools that help detect failures in the network, and will subsequently route traffic accordingly.  Most providers have redundancy on their network servers, to manage calls placed over their network.