I’ve tested Google Project Fi on my new iPhone XS.
Data services work great, voice service is functional (sans-WiFi / Data Calling).
SMS and MMS messaging seemed even more finicky than my iPhone 6 on project fi. At first, it appeared there was no SMS functionality at all iMessage wasn’t setting-up with the number and I couldn’t send or receive SMS messages. After restarting the phone, however, iMessage did register which indicates the phone was able to successfully send a background SMS message to Apple’s servers to confirm service. The SMS functionality remained, however, severely limited. SMS messages appear to send, however, they are never delivered to the recipient. MMS message functionality (as with any iPhone on FI) is completely gone.
The extra limited SMS functionality is quite odd as I am able to send/receive SMS messages on my iPhone 6 using Google Fi. I have attempted to troubleshoot this by adjusting the APN settings to h2g2 from the default of alpha as well as resetting the network settings. Unfortunately, this being an iPhone there just aren’t that many paths to troubleshooting an issue like this. As a side note, my Fi SIM card is pretty old so I will be ordering a replacement SIM just in case the newer SIM cards have better compatibility.
My original plan with this new phone was to use Verizon as the primary sim with Google Fi as a backup. Unfortunately, Apple does not yet support dual sim functionality, but it is anticipated it will be enabled with an update in November 2018. Upon availability, I will be using the built-in eSim for my Verizon service and I will use a pre-activated Project Fi SIM in the physical sim slot.