I use GV (which, I think they have been trying to rebrand as "Hangouts") in a spare that has a Fi SIM. When I travel to the US, I usually activate Fi for that trip. This allows me to keep my number, and is cost effective as long as don't need to use a lot of data (i.e. Fi data was a decent deal at $10/G when it first launched, but the price has never dropped).