Earlier quoted context omitted.
”Who even has a smartphone with an active data plan? Not homeless people” You might be surprised.
I’ve worked in outreach; the average homeless person has a smartphone but can’t afford data, only very basic prepaid call/text only top-up cards. And because they’re frequently losing phones (stolen, damaged by attacker, etc) they can’t keep a stable number, either.
Alternatively flip it around and the toilet scans your QR - this could even be a bit of paper. Ultra low tech.