See also, /r/weirdnumbers and, if you're into that sort of thing (that is, exploring the phone network), /r/phreaking
Old-skool phreaking[0] (eg, using blue boxes and red boxes) is mostly dead (at least in the US and most "first world" countries. Maybe there is some vestige of in-band signaling left somewhere else) but there is still some fun to be had exploring phones and phone networks.
Modern day phreaking is more about GSM sniffing[1], messing with the SS7 network[2][3][4], using SCTP[5]/SIGTRAN stuff[6], etc. etc. But, at least for the land-line / PSTN network, even some of the old "colored boxes"[7] still do useful things. You can always beige-box a landline phone, violet-boxes should still work, I think a gold-box would still work, etc.
If you want to dig deeper into how the PSTN works, a good, fun book is Understanding Telephone Electronics[8] by Carr, Winder, and Bigelow. Another interesting one is Digital Telephony[9] by Bellamy. Another "oldie but goodie" is Voice and Data Communications Handbook[10] by Bates and Gregory.
Also, don't ask me how or why I know any of this stuff... :-)
Old-skool phreaking[0] (eg, using blue boxes and red boxes) is mostly dead (at least in the US and most "first world" countries. Maybe there is some vestige of in-band signaling left somewhere else) but there is still some fun to be had exploring phones and phone networks.
Modern day phreaking is more about GSM sniffing[1], messing with the SS7 network[2][3][4], using SCTP[5]/SIGTRAN stuff[6], etc. etc. But, at least for the land-line / PSTN network, even some of the old "colored boxes"[7] still do useful things. You can always beige-box a landline phone, violet-boxes should still work, I think a gold-box would still work, etc.
If you want to dig deeper into how the PSTN works, a good, fun book is Understanding Telephone Electronics[8] by Carr, Winder, and Bigelow. Another interesting one is Digital Telephony[9] by Bellamy. Another "oldie but goodie" is Voice and Data Communications Handbook[10] by Bates and Gregory.
Also, don't ask me how or why I know any of this stuff... :-)
[0]: https://en.wikipedia.org/wiki/Phreaking
[1]: https://www.youtube.com/watch?v=wzyuioto4y8
[2]: https://www.youtube.com/watch?v=0JCusqL-Gdk
[3]: https://www.youtube.com/watch?v=-wu_pO5Z7Pk
[4]: https://en.wikipedia.org/wiki/Signalling_System_No._7
[5]: https://en.wikipedia.org/wiki/Stream_Control_Transmission_Pr...
[6]: https://en.wikipedia.org/wiki/SIGTRAN
[7]: https://en.wikipedia.org/wiki/Phreaking_box
[8]: https://www.amazon.com/Understanding-Telephone-Electronics-J...
[9]: https://www.amazon.com/Digital-Telephony-Telecommunications-...
[10]: https://www.amazon.com/Voice-Communications-Handbook-McGraw-...