According to the report, this is the reason why Fairphone were rated worst:
The last time we researched Mobile Phone, in 2016, Fairphone received our middle rating as it had made efforts to reduce the use of PVC, phthalates and BFRs. But in the research for this guide, Ethical Consumer could not find any information on the Fairphone website, nor did we receive any response from Fairphone, regarding their policies around these issues. For this reason, it received our worst rating.