One of the following ideas, in order of how likely I could get it running (given I wouldn’t have the foggiest what I’m doing):
Forward the 1-800 calls to a free VoIP voicemail service, inviting callers to leave to leave the date, their location, and a message. Print stickers with the number, slap them on however many payphones I can find, and see what happens. I could do this tomorrow if I wanted.
Same idea, but routing to a FreePBX set up with whitelisting. While slapping up stickers, dial an echo number (don’t think that’s the right term - one that just reads back the number you’re calling from, not one that echos what you’re saying to test latency), add number to whitelist. That payphone is now activated. Activated payphones get to leave a message, anyone else gets ‘Good bye’ and disconnected. Some reading suggests this is possible, but with many, many things to learn between now and then (especially whitelisting). I’d be starting from 0 knowledge.
The above, but when you hit # to end your message, you get access to some automated menus with some fun/weird stuff (qotd, show times for upcoming bands I find interesting, a party line would be cool, etc.). See all comments demonstrating ignorance.
Why? It’s pretty dumb, but seems like it’d kill some time and could garner some interesting/weird audio. I do like the idea of whitelisting payphones only, both to cut down on bot call vectors and to push the like 3 interested people to use the disappearing comms anachronisms around town.