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