The functions of the extra four DTMF tones (A, B, C, and D) missing from most phones have been touched on in other nodes, but I feel that a more complete description is in order. A, B, C, and D make up the 4th column of DTMF tones, combining a 1633Hz signal with either a 697, 770, 852, or 941Hz one. Most phones do not include them because they are of no use to the average person. For the curious, a device known as a Silver Box was devised to generate these tones.
On the (obsolete) Autovon military phone system, the four extra tones were used to assign precedence to phone calls. The levels of precedence are as follows:
- FLASH OVERRIDE (FO)
- Calls designated as FO will preempt calls of any other level of precedence. The capability of Flash Override is given to the President, the Secretary of Defense, the Joint Chiefs of Staff, CINCNORAD, and special military commanders for declaring DEFCON ONE or Defense Emergency.
- FLASH (F)
- Flash calls will override calls designated as immediate, priority, and routine calls. This precedence level is reserved for calls crucial to national survival.
- IMMEDIATE (I)
- Preempts priority and routine calls. This precedence level is for calls pertaining to the security of military forces, and civil defense.
- PRIORITY (P)
- Overrides routine calls, used for calls pertaining to government operations.
These levels of precedence correspond to A, B, C, and D respectively. Users of the Autovon were trusted to not request a level of precedence that is higher than necessary.
The ACD directs calls to the Directory Assistance operators. In the past, it was possible to access several hidden functions by calling directory assistance and holding the D button. If all went well, you would be greeted with a prompt (usually a pulsating or stuttering dial tone). Most of the functions are for testing purposes, and are useless otherwise. Pressing 6 or 7 at this tone would connect you to a loop circuit (that was probably monitored).
There were probably many other systems that used these tones for various purposes. Nowadays, they'd be pretty rare.
Excerpt from an Global AUTOVON Telephone Directory. http://www.undergroundnews.com/files/texts/underground/hacking/autovoni.htm
There May be Gold in that Silver Box. http://www.textfiles.com/phreak/BOXES/silv_inf.txt