Google Fi does some magical VOIP type things with your phone service, but it doesn’t play nice with Google Voice. You can’t have both Google Voice and Fi on the same Google account. (Here’s a Google Fi help article about the issue.) You also can’t forward your Google Voice number to a Fi number as you would with a normal carrier phone number. (I think there may be some workarounds for this, but don’t remember the specifics).
There are a few options:
Get Fi and transfer your Google Voice number to it
This is actually not a terrible option if you are going to use Fi long term as it has some of the nice features of Google Voice baked in. However, one of the things I like the most about Google Voice is the flexibility of being able to change plans which this option reduces. You can transfer your number back to Google Voice after leaving Fi, but it’s not the magical seamless process of not changing anything.
Get Fi on a different Google account with a different number
You can have two accounts on your phone and all the texting features of Google Voice will still work. To make phone calls you can use the Hangouts Dialer App to make them over data with your Google Voice number or use the native dialer to make them over non-data with your Fi number. This is a sort of ugly split-brain setup, but is fine especially if you are just using Fi short term for i.e. the amazing international coverage
Don’t use Fi at all. (This is the one I would choose.)
It’s actually not a super great plan apart from the international coverage. Plus all the hassle with Google Voice is not great.
You can find much better value/ $ plans. For instance I currently have the “AT&T Prepaid $50/month plan”. It’s actually $40/month if you sign up for auto pay and you get 8G, so as long as you use >2G and <8G/month it beats the Fi plan.
If you don’t think you’ll use that much data you can find MVNO plans that are “pay for what you use” in the same way as Fi, but for less. (“Ting” is the one that sticks in my head, but I haven’t really researched these plans. They tend to be relatively expensive per usage and only make sense if you think you won’t use much)
In general, check out r/NoContract for probably better phone plan recommendations.