This is my problem with Fi at the moment. i ported in a voice number but now if I pause service, I don't get SMS/Voicemail. So I have to keep the service on even when overseas for extended periods and using local sims - this way I get Fi calls to google hangouts on desktop.
Fi recently added a way to access the voice interface so I can at least see my pre-fi texts etc.
Why not just create a new Google Voice account? See below quotation from Google.
https://support.google.com/fi/answer/6246240?hl=en"If you want to keep using Google Voice
If you want to continue to use Google Voice with your current Google Voice number, you have a couple options:
Sign up for Google Fi with a Google Account that’s not linked to your Google Voice.
Transfer your Google Voice number to a different Google Account (as long as it doesn’t already have a Google Voice number) before you sign up for Google Fi. There’s no cost to transfer between Google Accounts.
If you use one of these options, you can use the Google Voice app on your Google Fi phone with your original Google Voice number.
Note: You won’t be able to forward a Google Voice number to a Google Fi number or forward a Google Fi number to a Google Voice number."