On a US trip and using a TMO SIM card which I still think is better than roaming on Vodafone. The card plan is $30(US) which provides a month of unlilmited data (well they say up to 5GB afer which you drop to 2G), 100 minutes of calls and unlimited texts. That compares well with $5/day Vodafone roaming which would cost me for $75 for 15 days.
And I don't care if people cannot reach my NZ number since those whom I care to know how to reach me can call my Google Voice number or use one of the many message apps I have and others can leave a VM.
Anyway I digress. I notice that TMO (as well as ATT Verizon and Sprint) offer WiFi calling. I turned it on on my Nexus 6 but for me it's not particularly useful since even with WiFi calling it still uses your plan minutes and I only have 100 which get used for both inbound and outbound US calls. So i make all my calls using Hangouts and provide only my GV number for people to reach me in the US rather than the carrier SIM number.
I can see if the carriers didn't deduct minutes from your plan to use WiFi calling but there is really no commercial inventive to do that.
Could be useful if you live in an area of poor cellular coverage but had good home WiFi so you could still make calls readily.