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.

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.

I want LTE gateway for home to redirect calls to SIP when travelling and not worrying about losing them.

I am digital nomad so have multiple sim cards that I need to maintain working and want to receive calls on, so just using VoIP number isn't exactly good option.