Most commonly it was handled like this: certain exchanges (the first three digits after the area code) at area-code boundaries could call each other without lon

Most commonly it was handled like this: certain exchanges (the first three digits after the area code) at area-code boundaries could call each other without long-distance tolling. The printed phone book gave a list of these. I knew of this at the Queens-Nassau counties border between the 718 and 516 area codes in New York. How widespread this was depended on how much the local phone company cared about being a good citizen to its users. And of course there were rules, exceptions, sub-exceptions, and so on (sometimes it worked in only one direction!) As with everything involving the phone network, it was an agglomerated mess over time, and yeah you can find examples of every possible case.