Thanks for checking in Mike.
Yes, that's what I have set.
I'm calling only US numbers.
Put the phone in Airplane mode then turn the WiFi back on. Be aware that outgoing calls on WiFi are charged as if called from the US. You don't normally want to call international or local numbers using WiFi. In fact, I'd use Hangouts, just to be safe.
I called support (+1-505-998-3793) from my mobile.
(I mention this bc I got disconnected as soon as I got hold of a rep.
He called back without getting my name or number, thought that was cool.)
Okay - I falsely thought that Wi-Fi calling meant that I was calling off of the network, therefore wouldn't be charged.
My rep recommended another solution which has been working great - Viber Out.
Unlimited calls to US-based lines for $2.99 a month, perfect for a traveler who spends much of the year outside of the States.
(Viber Out is laggy with some calls, others are perfect, and at $2.99 a month unlimited, it doesn't even matter).
Thanks guys for the support and I hope this helps someone in the future.