Dual Tone Multiple Frequency. Pressing a button on the keypad of a touch-tone phone generates a pair of tones of specified frequency and duration. The network or the equipment at the other end of the connection (such as a remote control for a phone answering machine, or a telephone interconnect system) detects and interprets these tones. In analog networks, audible tones generated by the network provide the call progress indications to the user. Different tones allow the human ear to interpret the progress of the call. On digital networks (such as PBX or ISDN), the network may send indication messages to the phone to indicate the status of the call, and the phone may generate most tones locally, driven by those messages.
