Project Fi offers a lot of flexibility when it comes to networks, with seamless switching between T-Mobile, Sprint and Wifi for calling, texting and data needs. While we've established that the switching isn't much to worry about and actually works quite well, the one shortcoming arrives when you go to make calls and use data at the same time on Sprint.
Just like other modern phones that are running on Sprint, there's no simultaneous voice and data support on the Project Fi Nexus 6. That is, if you're on the Sprint network and connect a phone call, your data connection will cut out — so you can't search for that restaurant you're trying to book together, or perhaps more importantly use your laptop that is tethered to the phone for an Internet connection.
While this isn't any big revelation to those who have used Sprint in the past couple of years, it can catch Project Fi users off guard. Without the assistance of an app you don't know whether you're on T-Mobile or Sprint at any given time, and you'll only find out that you're about to drop your data connection for a call right when you make or receive it. If you're on T-Mobile, the phone will keep a data connection, if you're on Sprint it'll drop out completely.
It's a hurdle that Google can't really do much about on Project Fi so long as it continues to let your phone automatically switch between networks, and Sprint's sluggishness of moving to VoLTE (Voice over LTE) isn't helping the situation. For now, your solution is to either make VOIP calls using the Hangouts app, or be ready for phone calls to potentially put a pause on your data consumption.