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...
Source: https://www.gsmarena.com/google_preparing_to_separate_fi_numbers_from_google_voice_numbers-news-41074.php
