Google Fi phones will accept any SIM card, they're not locked down at all. However, the Google Fi service will only work properly with designated phones. If y

Google Fi phones will accept any SIM card, they're not locked down at all. However, the Google Fi service will only work properly with designated phones. If you stick a Fi SIM in a non-Fi unlocked phone, you only get T-Mobile service. (I've done this with a backup phone - both an iPhone 7 and a Moto E4 - when my Pixel was being RMAed)