This is amazing. I would love to have this game in France! We have a geocaching scene (https://www.geocaching.com/, https://france-geocaching.fr/), but I really like the idea with payphones and this system of calling to claim findings.
The "love letter to a disappearing piece of infrastructure" bit makes me think of the payphone pictures that are published in each of 2600 magazine issues: https://www.2600.com/payphones
Another cool "just get out there" thing is the Degree Confluence Project. Just checked, and even the web site is still old school. https://confluence.org/
My personal contribution: https://confluence.org/confluence.php?visitid=3402
Anyway would love to play this payphone game, if only as an excuse for bike outings, but it's only for California and I don't live there.