'How can' is another part but what you seem to suggest ia a great idea in itself. We know that these discarded or old phones do not provide much value to their owners and can be bought at much less price everywhere. But, just gifting these phones in the hope of adoption shouldn't be the target. The target should actually be 'Bitcoin Education' before thinking all of this. I see you saud that people know how to recieve or pay in Bitcoins in many developing nations but there are also many people who still don't understand 'how to simply use a smartphone' for Bitcoin related stuff. I will put stress on Bitcoin education more because only then we can see a proper change.