Phone Numbers as Melodies
Because nature is full of harmonic sounds (sounds which
contain frequencies that are multiples of each other or that
are both multiples of the same base frequency), the
frequencies making up DTMF tones were chosen to be not
harmonic at all.
They are not even related to each other in the same way today's
musical tones are. You can make one or two of the DTMF frequencies
line up with musical notes (different pairs depending on how you
tune your instruments), but that's about it.
DTMF frequencies (left) vs Musical Scales (right)
While this makes the phone electronics cheaper to build and
more reliable, it makes playing musical tones with your phone
keypad somewhat difficult: phone keypads make really bad
instruments -- they are like a piano with 12 keys none of which
sound right by themselves, let alone played together.
Having said all this, many melodies can be played and are
somewhat recognizable. You should try it yourself next time
you call a friend.
WARNING: If you pick up your phone and start punching
numbers, you will probably get connected to somewhere and
you will pay for the call.