Secret? Okay, maybe not common knowledge vs. the # of people that use "normal" touch tone telephones (or used now, now that hardly anyone does!). But A/B/C/D are not a secret by any stretch. This stuff has been well documented for a few decades.
Back in the 90s when I first learned about this, and had a modem that would dial these additional digits just fine, I used to see how various PSTN LECs would handle them. Mostly ignored, but some results were interesting.
Also, back then I was living in the Greater Houston (Texas) area, and had to have a "metro" phone line to be able to get local calling to various NXXs outside of the core Houston (some even still came with a toll charge!). Often when dialing between these zones, the in-band MF tones used by the switching systems were audible to the caller.
A lot of this stuff really interested me, and I really got into phreaking and such as a teenager. See also red box, blue box, and all the other colors.
Back in the 90s when I first learned about this, and had a modem that would dial these additional digits just fine, I used to see how various PSTN LECs would handle them. Mostly ignored, but some results were interesting.
Also, back then I was living in the Greater Houston (Texas) area, and had to have a "metro" phone line to be able to get local calling to various NXXs outside of the core Houston (some even still came with a toll charge!). Often when dialing between these zones, the in-band MF tones used by the switching systems were audible to the caller.
A lot of this stuff really interested me, and I really got into phreaking and such as a teenager. See also red box, blue box, and all the other colors.