|This article does not cite any references or sources. (June 2008)|
A silver box is a modified DTMF keypad that adds the four additional keys used in the now-obsolete autovon phone system. This 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 in the Autovon system. (These keys were used to set the priority level of a call, and are the red keys in the photo on the right. More information is available in the Autovon article.)
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.