Strictly speaking, MF (the long-distance signalling tones) preceded DTMF (Touch-Tone) by decades. It was in use in parts of the Bell System long-distance network by the early 1940s. A failed attempt was made in 1948 to use these tones (generated by plucked reeds) for customer signalling. Touch-Tone wouldn’t be deployed until 1963, when transistorized oscillators could be made cheaply and reliably.
There is a fourth column in the DTMF standard, for a total of 16 possible tones. These were used for setting priority on military phone networks, and are used by ham radio operators to control repeaters. I think one of the Evan Doorbell tapes may have mentioned something about them also being used on early conferencing systems. If you have a phone capable of generating them, an Asterisk PBX can recognize them, and you could set up your system to actually use them for whatever you might come up with.
