Multifrequency

Patent Pending In telephony, multi-frequency signaling (MF) is a signaling system that was introduced by the Bell System after World War II. It uses a combination of tones for address (phone number) and supervision signaling. The signaling is sent in-band over the same channel as the bearer channel used for voice traffic. Multi-frequency signaling is a precursor of modern DTMF signaling (TouchTone), now used for subscriber signalling. DTMF uses eight frequencies.