MULTI-FREQUENCY

In telephony 'Multi-Frequency' (MF) is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a Multi-Frequency Sender, to be received by a Multi-frequency receiver in a distant telephone exchange. MF was used for trunk signaling, and is a precursor of modern DTMF "touch" tones now used for subscriber signaling.
MF signaling includes R2 signalling, R1 (in North America), and Signaling System No. 5.
For details on R1, see blue box.