A messaging failover is the automatic rerouting of a message to a secondary channel when the primary channel fails to deliver for example, switching from WhatsApp to SMS when a recipient has no internet connection, or from SMS to voice when a number is temporarily unreachable. Failover logic is the difference between a message that was sent and a message that actually reached someone. It operates invisibly when it works, and produces missed payments, missed appointments, and failed dispatches when it does not.
Most teams think about failovers as insurance. Something you set up in case things go wrong..