You do realise that you can do this for virtually any subject, right? And to be clear, with 'this' I mean go on endlessly asking for very specific definitions instead of attempting to understand what is meant by reading the context.
Juiceme answered most questions as I would.

Purism Librem 5 and Pine64 Pinephone both do this (see also this article), so I would say those two are good start for 'FOSS phones'. Although display and wifi/bluetooth drivers also seem to be a concern, often.

However, if you want to take it that far, there's not a single device out there that's runs on 100% open source software. Nor does there need to be in my opinion, companies have to right to execute their specific solutions (which they presumably worked hard for) in a proprietary fashion.

Hence my earlier defined understanding of what 'FOSS' phone means for me; runs 'mainline linux compatible', meaning it can run a recent version of the kernel (which is FOSS), drive the phone's hardware (which can partly FOSS), and run a GUI/DE (which can perfectly be 100% FOSS).