That and some information about the device, which IPs it is accessing and so on. A malicious firmware can periodically ping any server without the user knowing, which would allow the server to triangulate the phone via cell towers.
Discussion
Great,
So if this is your threat models just never use it at home - put it in a faraday bag & buy the pixel with cash.
Correct. But if you are at that level, I would also make sure there are no other electronics in your house. Anything can have a hidden modem these days, even your fridge.