Multi-frequency signaling

It uses a combination of audible tones for address (telephone number) transport and supervision signaling on trunk lines between central offices.

Because of the in-band transmission characteristic of MF signaling, the systems proved vulnerable to misuse and fraud by phone phreaking with devices such as a blue box.

The frequency combinations are played, one at a time for each digit, to the remote multi-frequency receiver in a distant telephone exchange.

The terminating office acknowledges the seizure with a ready state by responding with a wink start signal, which is a momentary off-hook condition.

[3] 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.