I believe, though haven't tested it myself, that more than likely the call will drop in that case, because the phone can't switch over from Wifi to cell fast enough. However, a tip I saw on Reddit recommends turning on a Developer Mode option to keep cell data on even when connected to Wifi, to enable fast handover.
Missed 3 calls on GV because phone went to sleep. Cannot trust VOIP. Need plan B. 
Update, noticed incoming call was turned off on Hangouts. Turned it back on, see how it goes.
Would you mean incoming call turned off in Google Voice? If you mean "on Hangouts", are you (and can you) run Hangouts in tandem with the new version of Google Voice?
No setting to turn off GV incoming call. Phone did not ring and I did not get a missed call notification either.
Incoming call was turned off on Hangouts... I might have turned it off from a prior setting.
I have both apps loaded on the same phone.
You should have settings to turn off incoming calls on your phone. On the GV app on lower left is the "calls" tab-- click on it>click on 3 parallel line in search bar top left>settings>scroll down to "calls" section>click on "incoming calls" and under "My Devices" you'll see all devices that will ring and can be answered or be turned off not to ring on incoming calls by toggling the switch next to the device. Or-- you can turn off the phone in your hand by going to call settings and scrolling down to "do not disturb" and turn it on to turn off message forwarding and send all calls to voicemail.
You can use both Hangouts and GV app on same device, but Google recommended that incoming calls to Hangouts be turned off.
Hangouts is voip like GV app.
Strange, turns out it was the phone I initially got the new GV app. When that phone is off, it goes to voicemail. I deleted that phone from my linked numbers.... works fine now. Still would like to figure out what caused the calls to go to voicemail when that one phone is off...
I believe if you have GV forwarding to more than 1 phone, a phone that is turned off will frequently 'grab' the incoming call and redirect to voicemail. That may be dependent on the particular behavior of the carrier of the turned-off phone.
If the phone in question (the turned-off one) was not designated to receive forwarded calls, I don't know.....
2 phones are under Tello. One was set to cell only and does not have the GV app and it does not send the call to voicemail. The other one has the GV app and was set on GV prefer wifi and mobile data, when off, it kicks the call to voicemail.
@PEW AFAIK, GV only ignores early media (eg; busy signal, "subscriber unavailable", etc.). The call should connect if "answered" by carrier voicemail. It may be possible to set up conditional call forwarding to GV voicemail, instead of carrier voicemail, for forwarding destinations. In legacy GV interface, this is called "Activate Google voicemail on this phone."
I never configured my Tello voicemail, so I don't think the 2 lines are treated differently due to voicemail setting.
Assuming both lines have carrier voicemail with default settings, it is strange your Tello lines apparently behave differently when powered off. To resolve this, you can set up conditional call forwarding to GV or to a number that always rings busy (early media) - eg; https://www.nthcircle.com/forum/tello/45-tello?start=150#1566
Missed another call, argh! I guess GV is just not reliable. Had the phone set to prefer cellular and the app set with the phone's phone number. The call went to voicemail without ringing my phone. Possible fix: Disassociate phone app of any phone number, set GV to forward to the cell #. So, device could potentially get 2 calls... just like when I had Hangouts and the cell phone both ringing on the same device. Bottom line, seems no improvement... only goal met is for Google to retire Hangouts.
Got an incoming call today. Looks like my fix worked.
At one time, texting through the Google Voice app did as Joseph describes, but if you had vour Google Voice text messages routed through Hangouts, it worked correctly.
As of the last time I was involved in a group message, it still does.
Edit: I just realized I forgot Firefox doesn't get tabs quite right, and this was the OLDEST thread I just opened, not the newest, so I kinda necro'd it. Sorry.
Edit 2: I also forgot pagination is below the quick reply box and answered something from the first page without reading everything in between. I'm on a roll today.