This was previously stated to be working on the G4 by the last poster in the thread this is branched from.
However with the latest software update - June 2017 - despite waiting after switching into airplane mode / wifi on, it always displays "can't make a call in airplane mode"
Verizon Support.... was this switched off on the G4 due to unreliability?
When I try to use wifi calling with wifi and mobile network on, it connects for a second or two to wifi calling then reverts to mobile network, which is very poor here.
We also have a G6 in the house that works much better, albeit with a 1-2 second drop-out every 5 minutes or so.
Verizon 2nd Line tech support imply that wifi calling isn't fully supported on the G4, without saying that in so many words. I'd be happy to accept that if a clear statement was made to that effect.
and we're rural, with variable / poor mobile network and good fixed wireless internet connection.