Yeah I know that call forwarding would be easier to setup, but unfortunately it's not always work especially for outgoing calls and it's exactly I wanted to research hardware solutions.
Actually idea with iphone isn't that bad. I just need to check if it's possible to setup multiple iphones to receive sms and calls.
Okay, it looks like this iphone feature only work if both phones on the same WiFi network and unlikely there is way to overcome it.
So it's typical Apple - implement some useful feature, but limit it to be completely useless for power users.