If you've been having the problem of Hangouts forgetting it's settings, and the 'incoming phone calls' box becoming unchecked, then give this a try. It's working great for me.
Have Hangouts, Dialer, and Voice installed on your phone/tablet, and have opened Voice and Dialer at least once.
Go to Google.com/voice on computer and CHECK the two boxes to have it forward to Google Chat AND to Mobile.
UNCHECK the 'incoming phone call' checkbox in Hangouts app on phone, or phones if you have more than one.
In the Hangouts settings in gmail on computer (hidden away on the lefthand sidebar) the checkboxes for 'sounds for incoming messages' and 'ring on incoming phone calls' should both be CHECKED.
When you receive a Voice call, your phone(s)/tablet and computer should all ring. In my case one phone is active on Verizon and the other is not on a carrier. Receiving a Google Voice call - or making one - should not use any carrier minutes, regardless of weather phone is active on a carrier or not.
I don't know if step 4 is relevant, but that's how mine is set up, and I think it may be important to update it to the 'new' Hangouts in gmail (on computer.)
I've taken many Google Voice calls over the past couple days, on both my phones, including the phone that's active on VZW when I'm AWAY from Wi-Fi, and still haven't used any carrier minutes.
When I'm at the house with both phones and the computer in front of me, and I get a Google Voice call, everything rings, but my Wi-Fi only phone rings in Hangouts, while my VZW phone rings in the 'normal' dialer. When I get a call on my VZW phone while driving, same thing, but yet, as strange as it seems, I still haven't used any of my carrier minutes.
If I hear from enough of you guys that what I'm talking about is impossible, I'll totally believe that Verizon just isn't updating my used minutes. But I feel like this is working as I've described.
I'll shut up now :-)
Share your results here!