If memory serves
You have a 4x4 grid with the numbers on, and each row and column has a frequency. So you play a two-note chord (of a sort) for each key-press.
EDIT: Hey, I was pretty close
en.wikipedia.org/wiki/Dual-tone_multi-frequency#Keypad
( ,
Tue 20 Jan 2009, 17:44,
archived)
EDIT: Hey, I was pretty close
en.wikipedia.org/wiki/Dual-tone_multi-frequency#Keypad