It's possible to transfer your number back to Google Voice after you cancel Project Fi: https://support.google.com/fi/answer/6079398 https://support.google.com

It's possible to transfer your number back to Google Voice after you cancel Project Fi: https://support.google.com/fi/answer/6079398 https://support.google.com/fi/answer/6079398 However, it's not guaranteed: "Google Voice’s number availability changes regularly, so even if your number was previously on Google Voice, it can’t always be transferred back. If you’d like to keep this number, we recommend transferring it to another carrier that supports it."