In telephony Multi-Frequency (MF) is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code. 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, and Signaling System No. 5.
For details, see blue box.