[kekebo]

Oh boy, this takes me back. This is from memory, so take it with a grain of salt: The calling cards I mainly used as a kid in the 90s were ones with a sim-like chip on it that you would insert into a phone booth until it ran out at which point you'd throw them away. Earlier/alternative ones worked, iirc, by calling a free phone line which prompted you to enter a valid calling card number which had credit assigned to it, and then you could make your call. If there was a way to reverse engineer the algorithm for valid calling card numbers (similar to a keygen) I guess you could hack it for free calls. If someone has a more plastic / accurate memory of this please correct me

[kekebo]

(Corresponding wiki entry for the remote version: https://en.wikipedia.org/wiki/Telephone_card#Remote_memory_s... )