Silver box

A silver box is a modified 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 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:

Silver box

A silver box is a modified 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 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: