Google Fi is an MVNO carrier in the US that runs on T-Mobile, Sprint, and a nation-wide network of Wi-Fi hotspots. Once you sign up for Google Fi, you need to either transfer your phone number over from Google Voice, or you must surrender that number. A single Google account was not able to support both services.
A Google Fi forum post from back in November announced that Fi is preparing the backend and taking steps to separate Google Voice from Google Fi. This means that Google Fi customers could benefit from a secondary Google Voice number while keeping a personal Google Fi number under the same Google Account.
All your voicemails are still stored by Google, but voicemails you received with on Fi are no longer available on https://voice.google.com. They’ll remain everywhere else you’re used to accessing them, such as the Phone app or other dialer apps on Android, or the Fi app on iOS.
A Google Fi user would eventually have two separate voicemail inboxes – one for the main Google Fi number, and a separate one for a Google Voice number. There are many Google Voice users who take advantage of the unique features like the ability to ring multiple numbers, the ability to access the phone from any device, or simply having a separate number. Voice users would now be able to sign up for Google Fi while keeping Voice features.