Hi there! Fairphone  aims to build a phone that is repairable and pays attention to responsible material sourcing and advocating for workers’ welfare. I think it would make nice hardware for Replicant. Whether this is technically feasible I cannot say. It runs the infamous  Qualcomm MSM/QSD platform, more specifically Qualcomm Snapdragon 632. Whether that model has an integrated modem I don't know.
For completeness sake, here are Devices evaluations threads for Fairphone 1  and Fairphone 2 
Thank you for working on Replicant.
RE: Fairphone 3 - Added by Jack K about 1 month ago
I'm not a Replicant developer, just a free software enthusiast.
That said, given my limited understanding, looking at https://github.com/WeAreFairphone/android_device_fairphone_FP3/blob/lineage-16.0/proprietary-files.txt it appears this SoC is not a good choice from a free software perspective, having far too many proprietary files, which would likely require lots of work to reverse engineer and/or configure existing free software to work in their place. There is a wiki page that explains the common issues with Qualcomm SoCs, and having many proprietary files is one of them - https://redmine.replicant.us/projects/replicant/wiki/QualcommSOCs
It is a pity. I wish Fairphone would choose better SoCs for their products, as I too value their ethical supply chain ideals.
All the best!