Yip, phoned the missus (Apple 6) yesterday, she was on wifi the phone coverage is bad for her at the office and me (S4) on CellC, crystal clear and no lag.
I received an unexpected whatsapp call on my iPhone 6 from a friend using Android last night (I still can't make calls out, still waiting for it to become available for me). It worked flawlessly aside from 2-3 seconds of lagginess in the beginning. I answered on wifi, talked a bit and then moved out of range of my wifi. The call seamlessly moved over to 3G. Brilliant!
Seems to be working.. when u click on the phone icon it makes a whatsapp call instead of cell call (5S). Was looking for diallers but in retrospect it was silly to think there would be a new screen.. there is the recent table tho