As a reminder, Fi is a MVNO or mobile virtual network operator that offers existing wireless networks to deliver a service, similar to Boost Mobile or Republic Wireless. The big difference with Fi is that instead of using only one wireless network – T-Mobile, Sprint and US Cellular. The idea is that it moves to the best-performing network no matter where it occurs.
The catch is that only a few phones are equipped with this ability to change carriers dynamically. While writing, these phones include all Pixel, Moto G6, LG V35 ThinQ, LG G7 ThinQ and Moto X4. If you do not have any of these phones, Fi will still work, but it will only depend on one carrier: T-Mobile.
I should explain that even before this announcement, you could get Fi to work with an unsupported phone, and yes, before I've used the Fi device on the iPhone. But there were some hoops that you had to change. One of them needed the right data settings, or some things just would not work, like MMS or web browsing. You should also run the Fi phone on your SIM card to start it, or it will not work at all.
But now it's no longer right. However, there are still some issues, especially those with an iPhone. At this point, iPhones Fi support is still a beta and it shows. According to Google, iPhones with Fi will be cut out of visual voicemail, can not call or write via Wi-Fi (iMessage is released) and can not be used as a data access point outside the United States. Like many other non-existing devices, the iPhone also can not change the carriers dynamically. Also, if you have an iPhone 5, 5c or older, you are not completely successful.
I shipped the Fi SIM card to my iPhone XS and I am launching the Google Fi iOS app today. The app gave me enough warning that iPhone did not support all of the Fi features and then went through the changes that I had to take to make mobile data settings. I changed the APN (access point name) values to "h2g2" and entered the MMSC (Multimedia Messaging Service Center) in the new URL as indicated, and voicemail I was able to send and receive text messages as before. I also got to the web without a big problem. However, here I note that Google said it may need to adjust these settings as often as an iOS update is available. In addition, you can also use the Fi application to test data usage as well as monthly reports.