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...

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).