What are the ABCD tones?

The ABCD tones are simply additional DTMF tones that may be used in any way the standard (0-9) tones are used.

The ABCD tones are used in the U.S. military telephone network (AutoVon), in some Automatic Call Distributor (ACD) systems, for control messages in some PBX systems, and in some amateur radio auto-patches.

In the AutoVon network, special telephones are equipped with ABCD keys. The ABCD keys are defined as such:

A - Flash
B - Flash override priority
C - Priority communication
D - Priority override

At one time it was possible to use the ABCD tones to access a special maintenance mode of the Automatic Call Distributor (ACD) systems used by Directory Assistance operators to connect two callers together.

The purpose of the Silver Box is to create the ABCD tones.