[matthiasl]

You're right; I think CCITT5 is just another name for SS5, because different groups were writing standards. Bell called it one thing, CCITT (an international standards group) called it another thing. And then in the 1990s, the CCITT renamed itself to ITU.

[plapsley]

Yes. See https://en.wikipedia.org/wiki/Signaling_System_No._5

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.

[plapsley]

CCITT (ITU-T) version can be downloaded for free (!) from their website. https://www.itu.int/rec/T-REC-Q.140-Q.180-198811-I/en Specific download link in English is https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.14...

[jasomill]

Fascinating article. See also

https://web.archive.org/web/20120314023131if_/http://www.alc...

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).