Originally Posted by Need

Yes, I got the same thing, but I contacted Google Fi and you can get around it. The reason you lose the GV number is because that GV number is associated with your Google Account that was used for Google Fi. The Google account you used to join Google Fi cannot have 2 GV numbers: the GV number AND the mobile number you are porting in. So what you need to do is to create another Google Account (or find another existing google account without a GV number). On that new Google Account, opt in to Google voice term and service. Now go to your GV account on the Old Google account and "transfer" (There is option for that) your GV number to the new Google Account. Once transferred to the new account, your GV will be saved when you port in your mobile number to the original Google account.

Now you just need to update your hangout app to use the new Google account. Keep in mind that new GV account won't have your call/voicemal/message history. They still show up on your old account, but to manage the GV number, you need to use the new account.