Other than another failed attempt on google's part to unify disparate things (like google and youtube accounts), I don't know why fi hijacks your gv number. Fi's page is also so uninviting. I have been casually interested in giving fi a try, for example to buy a phone like moto x4. If you go anywhere on fi's page (https://fi.google.com/about/phones/ for example), it won't let you do anything without a google account, which I know will hijack gv. Just why ? Let me buy a phone, and make calls, and I'll give you money. But no. And because of this unnecessary mess with gv, I would never try fi.