A silver box is a modified

DTMF Dual-tone multi-frequency (DTMF) signaling is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed ...

keypad that adds four additional keys. This gives four columns of four keys each (16 total) instead of three columns (in the standard 12-pushbutton handset). In the now-obsolete

Autovon The Automatic Voice Network (AUTOVON, military designation 490-L) was a worldwide American military telephone system. The system was built starting in 1963, based on the Army's existing Switch Communications Automated Network (SCAN) system. I ...

phone system these keys, used to set priority of a military call, were the red buttons in the photo on the right. Autovon included four precedence levels: Routine (no special tone), Priority (D), Immediate (C) and Flash (B) with Flash Override (A) as a capability. Each had the ability to interrupt lower-priority calls in progress if all trunks were busy. Each was activated using a button in an additional column of the keypad: * A (697+1633 Hz): Flash Override (FO) * B (770+1633 Hz): Flash (F) * C (852+1633 Hz): Immediate (I) * D (941+1633 Hz): Priority (P) Autovon was replaced in the early 1990s by the

Defense Switched Network The Defense Switched Network (DSN) is a primary information transfer network for the Defense Information Systems Network (DISN) of the United States Department of Defense. The DSN provides the worldwide non-secure voice, secure voice, data, fac ...

; much of its infrastructure is now dismantled.

Amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency radio spectrum, spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emer ...

equipment continues to be manufactured with 16-key DTMF keypads, keeping extra tones available for on-air use to control remote apparatus such as radio

repeater In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Some ...

stations. These tone pairs (labelled A, B, C, D) are rarely used. Conversion of twelve-button keypads was usually accomplished with the addition of a toggle switch and a crystal that switched one column of a standard phone keypad into the "fourth column" used to generate 1633 Hz as the higher of the two tones output on a keypress. Modern phones with an integrated circuit based DTMF generator can frequently be modified by simply soldering a wire from the 1633 Hz leg to a switch that toggles between that leg and the 1477 Hz leg for the rightmost column of keys.

References

{{Phreaking Boxes Phreaking boxes