Comment by plapsley

Hey Fogus! A few links or resources you might enjoy:

* Nationwide Operator Toll Dialing, 1945: https://explodingthephone.com/hoppdocs/nootd1945.pdf

* General Switching Plan for Telephone Toll Service, 1930: https://explodingthephone.com/hoppdocs/gspts1930.pdf

* (Book) Engineering and Operations in the Bell System, 1984: https://bitsavers.org/communications/westernElectric/books/E...

* (Shameless plug) My book on the history of phone phreaking, Exploding the Phone, which has a lot of stuff on Engressia in it: https://explodingthephone.com/

Also, there is a documentary film coming up at Sundance about Engressia! https://festival.sundance.org/program/film/6932fad21a5535277... Very excited to see it!