MrWizard has it right. Some of the new T-Mobile phones are set up that they can use WiFi just like a VoIP type of service. The phone just has to be connected to WiFi to do it and it doesn't matter that the DATA is coming from a 4g hotspot from Verizon, as long as the data speed is fast enough.
I use T-Mobile cause they actually offer an unlimited 4g data plan and I can stream movies all day if I want, but yes I am usually near a city. I plan to do what the OP is doing and get a Verizon MiFi (or similar) to be able to have at least some data when in the boonies.
Remember though that if going this route to keep the calls short as the data is still being used fairly fast.
