If I were you I would create a different Google account for fi. Get a new # assigned with that account. Keep your GV number as a GV number and use it simultaneously with the fi number. You wouldn't even have to give the fi number of to anyone and just use your GV only thru hangouts dialer.
For example, I have my GV number and my fi number (my ported cell carrier number) on two different Google accounts. Both Google accounts are signed into this phone. I can make and receive calls and texts from both numbers from the same phone.