I mentioned this article to my retired AT&T engineer wife, and she blurted out "Robbed-bit signaling!"
I just thought to post this since when I visited in September, some actual retired telecoms engineers were part of the tour group. One had been involved in the development of submarine cable technology.
Out of all of those however, panel is my favorite however, it sounds amazing to listen to, both the equipment and what the line sounds like. A call between a panel and the number 1 xbar sounds amazing, nothing quite sounds like revertive pulse signaling.
Interestingly enough, there were three switches made that used RP internally - Panel, 7A Rotary and 1XBAR - and if you go find video of the 7A Rotary, you'll see it sounds almost exactly like panel, because the unique sounds of the rotary sequence switches both exchanges use.