I've been WiFi calling with GV and Hangouts for over 5 years. For me, all voice, video calls, and text run through Hangouts. We use Ting to access the T-Mobile network. But I only get charged when using the mobile data network, which is relatively rare. The vast majority of the time, I'm in a WiFi network. My phone has a number assigned by Ting but I have no idea what it is. I disabled all voice and text functionality using 'device setup' on Ting's website. The only number people know is my GV number. It's also nice to sit at my desktop PC and make/receive calls, text, etc using the Hangouts extension for Chrome. And it's free to call the US when I'm traveling internationally.
DW also has a GV number that rings Hangouts. But she primarily uses her regular Ting number. I set up conditional call forwarding for her, such that incoming calls are forwarded to her GV number on Hangouts any time she's in an area with no cellular signal. She's at her parents house a lot these days and the T-Mobile signal is very weak at their house, usually no signal at all. So this works well. Her phone (Moto G4) does not support conventional carrier-provided WiFi calling .