Activate call forwarding when not registered - failover

[Riom]

Is there any way to set a call forwarding rule to be activated when my SIP device is not registered with Voxalot? This is essentially failover.

I have a Nokia E51 phone, which connects to the voxalot account via wifi in my two main locations. When I'm out of range of either wifi network (when the phone is not registered), I'd like incoming calls forwarded to the mobile number instead (from the voxalot account, using my dialling rules).

Is there any way to do this? I did see a message on here from late last year about failover being in development, but can't find a more recent reference.