Key pulsing

In telecommunication, a key pulsing (pulsing) is a system of sending telephone calling signals in which the digits are transmitted by operation of a pushbutton key set.

The type of key pulsing commonly used by users and PBX operators is dual-tone multifrequency signaling. Each pushbutton causes generation of a unique pair of tones. In military systems, pushbuttons are also provided for additional signals, such as precedence.

Source: from Federal Standard 1037C and from MIL-STD-188