> that change was held in some kind of side compartment so the same coins inserted could be returned That's exactly how it works. > or there was a way for the

> that change was held in some kind of side compartment so the same coins inserted could be returned That's exactly how it works. > or there was a way for the payphone to make change. It was controlled by a relay. If the CO pulled the relay to bill the call, the coins would divert into storage, otherwise the coins would fall into the return. If the signal was weak or was intentionally blocked then the coins would always fall into the return. It's possible that users just didn't notice the coins falling into the wrong slot. See the History section here: https://en.wikipedia.org/wiki/Green_box_(phreaking) https://en.wikipedia.org/wiki/Green_box_(phreaking)