Wow! In that case, a phone could mine at least a few satoshi per year, assuming that there are no costs whatsoever.

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I can't find any actual useful information on this topic, but it's most likely that these would be used to mine altcoins rather than Bitcoin.  They would have to be running on ASIC-resistant coins or they would still be essentially worthless.

There is something to the concept of reusing old smartphones though.  From a business perspective it's been overlooked, because I think that people would be more willing to pay the high prices for a good phone if they knew that they could resell it later for a decent price and it would still be used.