By default all extns should be created to use FQDN - not IP address. Failover requires everything uses DNS - however 3CX defaults to creating extns configured to bind to an IP address. This will not work for failover. In our instance it's defaulting to the IP of the separate SIP trunk NIC which the phones can't even connect to. Extns have to be manually created / changed to use FQDN - this is "sub-optimal". See case #96379. If there's a valid reason the default can't be changed then the failover configuration process should check this - because otherwise it won't work.