Good question. I don't think Axon supports this feature yet.
One way around this might be to install a softphone as an Axon extension that is set to auto-forward calls to another number. Then you would set-up a group in Axon for incoming calls that would call both your normal extension and the softphone extension that forwards to your cell.
I'm not certain, but I think NCH's Carousel software might work. I believe it can register with Axon as an extension, take incoming calls and then forward them to another number.. although I'm not sure about this at all and that's certainly not what Carousel was built for, I'm just trying to think of workarounds at this point.
Edit: You can also configure an extension to forward to another number after a certain amount of time, so extension rings for 10 seconds and cell rings for next 10 seconds after that as an example. Not the same, but easier to set-up.