A modern payphone, I think, would be an iPod Touch (or other equivalent non-cellular mobile device), available at the counter at bars/restaurants/hotels/etc., locked into an app that lets you tap your credit card to place a call. I could see Skype getting into that market.
(The modern spiritual replacement for a payphone, though, is just asking random people on the street if you could borrow their cell to place a call. You pay in status and social capital.)
ironically, every self respecting hacker would be ok if he was trhow in jail for that. He didn't invent it or anything close to that, but tried to sell and make money. lacking a proper car analogy, akin to a script kiddie advertising malware he found on some BBS of later.
I think I was around 13, which would have been 1997 - a red box was a simple thing to build if you had any sort of soldering supplies, and moderately fun to play with. I didn't even really have anyone I needed to call - I think I probably used it maybe three times in my entire life.
But that is interesting. The 80's system creators, probably let the authentication (rate and money counting) on the backend. And didn't filtered input probably because they thought anyone willing the cheat would just cut the wires and bypass any filter on the phone to send the tones... so in the end, the whole system was based on the tones being secret.
The 90's system assumes anyone cheating would be deterred by the sofware on the phone, and leaves an open line? So it is based on everyone using the phone? what happens if someone bypass the phone and go to the wire directly? is there a tone?