Shortly after my DTMF post, I was asked about the A-D keys.  As I mentioned, the DTMF standard contains high and low frequency for four additional keys - named A through D.  While these keys are in the standard, no current phones can simulate these keys.  These keys were originally created for military purposes - probably for some military type of phone - but the phone never came to mass production.  Although the phone was never produced, these keys still made their way into the standard.  At first thought it seemed to me that useful applications could make use of these "secret" keys but the reality is support for these keys in telephony hardware and software is spotty at best.

Update - it looks like I was mistaken about these tones not existing anymore (see comments).  While it is true that "normal" phones do not use these tones, they are still currently used by the US military.  The tones indicate the priority of the call.  They are as follows

A - Flash

B - Flash priority override

C - Priority communication

D - Priority override

It is also not true that only the military uses these messages.  The tones are used for control codes in some PBX systems and within some types of telephone networks.  Several years ago these DTMF codes were also used by telco companies to enter special administration areas where you can connect two callers together.  Phone hackers would create what is called a "silver box" to simulate DTMF codes and hack into this feature.  I imagine this was one way to have two people "call" each other and listen in on the result.  Unfortunately for those of you dying to try this out most telco networks no longer support this - probably because it was too easy to hack.  Aaah, the things I wish I knew as a kid. :)