# Silver box

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For the play by John Galsworthy, see [*The Silver Box*](/source/The_Silver_Box).

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A diagram of an Autovon telephone keypad with the four precedence levels

A **silver box** is a modified [DTMF](/source/DTMF) 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](/source/Autovon) 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](/source/Defense_Switched_Network); much of its infrastructure is now dismantled. [Amateur radio](/source/Amateur_radio) 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](/source/Repeater) 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.[1]

## References

1. **[^](#cite_ref-1)** ["AOH :: Phreaking Boxes - Silver, White :: SILVER2.TXT"](https://web.archive.org/web/20110704200005/http://artofhacking.com/tucops/phreak/boxes/silver/live/aoh_silver2.htm). 2011-07-04. Archived from [the original](http://artofhacking.com/tucops/phreak/boxes/silver/live/aoh_silver2.htm) on 2011-07-04. Retrieved 2023-11-11.

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Adapted from the Wikipedia article [Silver box](https://en.wikipedia.org/wiki/Silver_box) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Silver_box?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.