Also, in-band signalling between switches went the way of the dodo. SS7 sends signalling via a separate channel from the audio path. The transition began in the 1970s with CCIS, a predecessor that Bell Labs developed as they were designing the #4 ESS long-haul tandem switch.
These days, a lot of phone traffic, even at the carrier level, is now VoIP as well.
There’s quite a treasure trove of recordings, both raw and narrated, of 1970s-style phreaking over at the Evan Doorbell website.
