I agree the analog extension and pickup group setup is the best route. In some software levels there was a bug that if a line was programmed to ring on two extensions the call would follow the forwarding of the highest port number regardless of timer programming. If you program a vms key with the sub code of the analog extension on her phone she will be able to see when there is a messge in the other box.

If your not having fun, go find something else to do.