WiFi calling isn't coming to Verizon for awhile so the S5 may not even support it when that time comes.
As for how the WiFi calling will work I am sure only calls to people within the US will be free ... Such as with T-Mobile if you're out of the country you can call back to the US for free but you cannot call the world for free via WiFi calling. I am sure Verizon will not allow free calls everywhere in the world since they make $ from people wanting to call out of country.