One possibility is to use a VoIP provider that has failover options.
Basically this means if the call can not be put through to your Axon box for whatever reason (e.g. system down time, power failure, Internet disruption, etc.), calls can be forwarded to another number instead.
One provider that offers this feature free of charge is Voip.ms however last time I tried it didn't work so well with Axon (although I think NCH may have corrected this in the latest version).
Voip.ms offers virtually unlimited incoming for local Australian numbers at a rate of $10 USD/month which handles up to 2 simultaneous calls.
They also offer Australian toll-free 1800 numbers for $10/month plus 6 cents/min and they handle up to 50 simultaneous calls
At this time there is no porting option for Australian numbers however.
I'm sure there are probably other Aussie providers with failover, I just don't know which ones. You can try Faktortel, they seem catered towards business users and they may have some failover options.