I've never used Google Voice, but I do use Google Fi, and they offer unlimited calls and sms in what looks like every country on the planet. What's the advantage of Voice for the traveling caller?
Fi has started to threaten that they will cancel your account if you are overseas for extended periods a month, maybe two or three. So far it seems to reset if you usage is low and you visit the US often enough, but if you want to take an extended trip, it may simply stop working at some point.
The error message says you need to stay in the US, with "significant usage" for at least 30 days for access to international data to be restored.
I typically visit the USA a few times per year for a week at most (which is why I have the Fi plan in the first place), so this message has been on my account for over a year, and so far I have not seen any indication that Google is going to cancel my Fi plan.