Way back in TELECOM Digest Volume 12 Issues 627 and 628 (and a couple of others in the vicinity), several readers commented on what the A/B/C/D tones did on their various telephone lines. The only reason I remember is that I was one of the ones who commented. The general consensus was, it depends on your switch. In my case, a friend and I had found that, on our switch, after activating three-way calling and getting a dial tone, pressing C immediately terminated the second dial tone and bridged the original two parties back together again. Other people reported a few other oddities. Nobody reported anything too mind-blowing (didn't reach anyone who answered the phone, 'Yes, Mister President!' or the like).
John C. Fowler, snipped-for-privacy@yahoo.com
[TELECOM Digest Editor's Note: I wonder if I should hunt down those two issues from the archives (off hand, sounds like 1993 or 1994) and print them again ....? PAT]