TONE or DTMF (Dual Tone Multi Frequency)

The tones produced by your touchfone to signal dialled numbers to the PSTN, when tone dialling is selected.  DTMF frequencies are standardised - a pair of frequencies are uniquely linked to each digit ( 0 – 9, and * or # ) on the telephone keypad.

The 'dual tone' part of DTMF refers to the fact that each key press results in a product of two frequencies depending on the row and column of the key.