This topic made me curious and I tried looking into it a bit further. I found a lot of rankings of different mobile brands and electronic companies, two of which I listed below:
According to these two lists Nokia seems to be the "most" fair of the big electronic producers for feature or basic phones out there. A lot of articles I found regarding the topic of ethical/sustainable/green mobile phones actually linked back to these lists, so they "reported" the same results.
There's another listing I found, that focuses more on mobile phones:
Nokia does not fare well in this ranking, however "Doro", "Amplicomm" and "Alcatel" do. I didn't want to subscribe to the site though, so I couldn't see exactly how they scored their rank. But all of these companies produce feature or very basic mobile phones. Maybe this is something for you to further look into? I think this is the closest to a fair feature phone you could get at the moment.
@Stefan I found it quite hard to trade my old qwerty-phone for a full touch-phone! I would've used it till it died on me altogether if it weren't for Fairphone's overall concept convincing me. But I guess that's a different kind of discussion..