VoIP glossary
What is Failover?
Failover is the automatic switch to a backup path, server or carrier when the primary one stops working, so calls keep flowing without manual action.
Failover explained
In voice there are two kinds worth separating. Signalling failover means your PBX has more than one SIP server address and moves to the next if the first stops responding, often using DNS SRV records or SIP OPTIONS keep-alives. Route failover happens inside the provider: if a downstream carrier rejects a call, the switch retries on the next carrier for that destination.
Failover only helps if it is tested. Configure OPTIONS pings with a short interval, check that the backup address is actually allowed on your firewall, and make one test call through each path before you rely on it.
Example