'The Strone Roam is a smart device that allows you to make and receive calls and messages on your phone in another country while eliminating roaming charges. Simply insert your SIM card into the Roam and leave it at home. While away, calls and messages will find you on our Smartphone App.'

So the active sim stays at home. It needs wifi to forward any calls. Then at your destination you need wifi (internet) as well to receive them (or make them).

Interesting. Although I am not sure why I would want to keep my mobile number. A similar setup for landline numbers in any country is much easier (needs no hardware and no safe location with wifi).