Just to be clear, with the hotspot option in Yosemite, there is now free tethering between the iPhone and a Macbook? How come the phone carriers aren't raging like they did when iTether was released?
wifi yes, free. but outside wifi, you would need data to tether..
here's the way I look at it. basically what's going on is you're idevices, MacBook and iPad.. are tapping into the internet connection of your phone. Hence if you're on wifi, you can use wifi calling via internet.
if you're outside wifi, you would need a connection, LTE etc, to use this feature. The only way to do that is by tethering via your phone/data plan.