I posted late last year asking for something similar - I want to forward to my landline if my internet connection goes down.
Take a look at the thread as I did find a way to set up my landline with pbxes as an extension of voxalot for incoming calls so it rings at the same time as my voxalot internet connection. I ran into problems though as pbxes was answering incoming calls with an error message saying "all circuits are busy" so I stopped using it.
http://forum.voxalot.com/voxalot-sup...k-service.html
This would be a really useful feature for me but things seem a bit slow now though when it comes to new features.