I guess it would be too much overhead, but maybe it is somehow possible:
A mixture of lottery and saving until the price of a phone is reached. Every month you can pay 5,50 Euro or multiple of it and you get one lottery ticket, no matter how much you spend. 5 Euro get saved for your FP, 50ct are thrown into a draw pot. Everybody can spend money to the draw pot, of course. If you win, your savings get part of the draw pot. If you haven’t won until your savings reached the price of a FP4 the money can be used for buying a FP at the FP shop.
Fairphone could spend half a FP and as soon 290 Euro are in the draw pot one FP gets raffled. The savings of the winner get part of draw pot for the next month. In case there is enough in the draw pot for 2 or more phones then more FP will be raffled.
Some german bank provide Gewinnsparen (literal: “Winnings-Saving” german WP [1]). Maybe that can be copied. I guess a lottery license is necessary. Maybe a bank can be used as a partner to manage the money and organize the lottery. Then some money would have to be spend to the bank.