If you've got an older phone on Google Fi, you get paid to upgrade to a newer device.
Google Fi is offering a $100 bill credit to customers with non-Voice over LTE (VoLTE) phones to upgrade to a newer model. Google has told 9to5Google that to qualify, you'll need to have a non-VoLTE phone that was active on Fi as of December 3. You'll get an email from Google if you're eligible for this promo.
Also of note is that to get this $100 bill credit, you must buy your new phone from the Google Fi store and activate it before December 31.
The reason that Google Fi is offering this $100 bill credit is because T-Mobile will require all phones on its network to support VoLTE in January 2021. Google Fi is an MVNO, or Mobile Virtual Network Operator, which means that its service actually runs on T-Mobile's network, and so Fi customers will need a VoLTE phone just like T-Mo customers will.
Google Fi will stop activating or reactivating phones that don't support VoLTE in January. Google says that existing devices that don't support VoLTE will continue to work in the "near term", but it hasn't give a hard cutoff date yet. If you've got a non-VoLTE phone and Google is offering you the $100 bill credit to upgrade, you've got a few weeks left to take advantage of the deal.