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.
Digits are represented by two simultaneous tones selected from a set of five (MF 2/5), six (MF 2/6), or eight (MF 2/8) frequencies. The frequency combinations are played, one at a time for each digit, to the remote multi-frequency receiver in a distant telephone exchange. MF was used for signaling in trunking applications.
Using MF signaling, the originating telephone switching office sends a starting signal such as a seizure (off-hook) by toggling the AB bits. After the initial seizure, the terminating office acknowledges a ready state by responding with a wink (short duration seizure) and then goes back on-hook (wink start). The originating office sends the destination digits to the terminating switch.
MF signalling tones were vulnerable to being spoofed using blue boxes which generated a 2600 hertz tone to disconnect a toll call in progress and provided an operator-style MF keypad to dial another call using the same trunk.
MF and other in-band signaling systems differ from Signaling System 7 (SS7) in that the routing digits are out-pulsed in MF format in the same voiceband channel used for voice. The dialing user cannot detect these digits being out-pulsed because the audio connection is not established all the way to the user’s handset or device until after the connection is established with the terminating switch. Following a full connection, the same audio channel is connected to the user in order to communicate the voice, modem or fax data across that same 64-kbit channel previously used for the in-band MF signaling.
In-band signalling began to disappear as electronic switching systems displaced mechanical switchgear.
Out-of-band Common Channel Signaling (CCS) became nearly universal at the end of the 20th century in the United States. Benefits include higher connection establishment rate, better fraud security and features such as Caller ID
Some 911 Public Safety Answering Points (PSAPs) still use the MF format to identify the calling party to the PSAP when processing calls from Mobile Telephone Switching Offices (MTSOs) and land telephone offices. This is based on an earlier system which used MF to identify the calling party to a feature group 'D' (101xxxx) alternate long distance provider.
Other countries may still use a version of in-band signaling.[which?]