Google Fi Voicemail

Straight from Google Fi site:

Fi works on this iPhone®, but it's in beta

Fi for iOS is in beta, which means you won't get 5G data and some features need extra attention. You'll check your voicemail in the Fi app, and you'll need to adjust your phone settings to set up texting (we'll walk you through it).

Here is an article that may help.

• Set up Visual Voicemail on your iPhone - Apple Support
  • See the section, Get help with Visual Voicemail, for more information.

Voicemail is a carrier feature.

Contact Google for help with their service.