The 'Backup SIP server' will be used for outgoing SIP calls when the 'Primary SIP server' is not reachable and the SIP failover is enabled. By default, SIP failover is disabled.
When the failover is enabled, if there is no response at all or if a SIP 4xx is received, the fallback will be performed to the 'Backup SIP Server' after 'Failover timeout(seconds)' . As soon as a SIP 180 or 183 or ISDN ALERTING is received, the call to the destination is possible and no fallback is performed.
This field must be a valid IPv4 or a fully qualified domain name (FQDN). If a syntactically incorrect value is entered, the field will be marked as invalid and a warning will be displayed the first time when attempting to change tab. Trying again to change tab will work and the gateway will attempt to use the value entered. The check is purely syntactical and no pinging of the address is done.