I just came back from the The National Museum of Computing (TNMOC) in the UK at Bletchley Park. It was fantastic. They have a wide variety of working systems that you can play with, from home computers to mainframes, as well as Bombes and the Colossus (used to break the Lorenz cipher), and WITCH. Very cool. https://www.tnmoc.org/
Quasi-related: If you're into old telephone switching equipment (some of which is computer-based), the Connections Museum in Seattle is not to be missed.
Rachael Morrison and her team poured their heart into this and it shows... it is just so good. (Disclaimer: I was interviewed for the movie and narrate parts of it.) See it if you can.
Rachael Morrison, the director, was kind enough to invite me to the premier at Sundance, since she interviewed me for the film. She and her crew did an amazing job. She found so much archival material and she and her team stitched it together to tell a moving and historically accurate story. People were in tears after the premier. Just so good.
* (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/
Evan's stuff is excellent and worth watching/listening to.
Shameless plug: if you like his stuff, you might also like my book, "Exploding The Phone", which is a history of phone phreaking. https://explodingthephone.com/
SS5 was derived from AT&T's US MF signaling system, described in "Signaling Systems for Control of Telephone Switching" by Breen and Dahlbom, Bell System Technical Journal, November 1960. PDF here: https://explodingthephone.com/hoppdocs/breen1960.pdf
The BSTJ article has a discussion on international signaling on pp. 1430-1441.
for an early technical article describing the implementation details of the familiar DTMF "touch tone" dialing system, noting that the precise details differ from the final implementation — in particular, the high group frequencies increased from 1,094/1,209/1,336/1,477 Hz to 1,209/1,336/1,477/1,633 Hz, possibly to mitigate the "pulling" effect described on pp. 251–252 (though I can find no reference for the rationale).
Yes! This is what I concluded when, a few years back, I did a similar analysis to what matthiasl posted. <shameless plug> If you're interested in this kind of thing, please check out my book, Exploding the Phone. https://explodingthephone.com </shameless plug>
Someone mentioned phrack above. There was (still is!) also 2600, and before that, YIPL/TAP, the original phone phreak newsletter: https://archive.org/details/YIPLTAP_1-91
Quasi-related: If you're into old telephone switching equipment (some of which is computer-based), the Connections Museum in Seattle is not to be missed.