I use Google Voice (not Google Fi) for most of my calling in the US. Google Voice gives me a second US phone number (free), but it is not exactly a phone service. You need existing phone service to be able to sign up and get a second number. I could forward my Google number to my regular cell number if I wanted to, but I don't. Instead, I receive calls on my Google number in the Google Voice app on my phone. I can make calls to US numbers too. Receiving and making US calls is completely free - or at least, I mean, there is no cost to use Google Voice. I need to be on WiFi or use mobile data (and mobile data isn't free).
When I'm in Europe, I use a Dutch Vodafone SIM for data for my phone. I can still make and receive calls to US phone numbers for free with the Google Voice app. (It's not free to call European numbers with Google Voice - costs a few cents/minute and you need to buy credit in $10 blocks. I would probably try WhatsApp for calling Europeans since it's very popular there.)
So you can always get prepaid SIM cards like mine to use in Europe and make/receive calls with Google Voice like I do. You don't need a SIM AT ALL to use the Google Voice app if you are on WiFi. But you need to have signed up for Google Voice ahead of time, while at home, using your US phone service. Presumably you will want mobile data anyway so either a SIM card like mine or some US phone service that offers international roaming. You could always switch to T-Mobile, AT&T, or Verizon so you can do seamless international roaming while in Europe - calls to US numbers are not free, but then you could use Google Voice for calls - completely free for you to call US numbers and if you are on WiFi, no data use either.
I can't guarantee you can RECEIVE incoming calls in the Google Voice app while you are in Europe. It worked for me; Google associated my Dutch Vodaphone SIM with my Google account automatically, and I started receiving calls. Since my Google number is the only one I give out anyway, it worked the same for me in Europe as at home.
No harm in signing up for Google Voice right now even if you never use it - it's free. If you wait until you get to Europe, it may be too late to sign up for it. Try Google Voice now and learn how the app works.