Elektor Electronics November 1994
has an article in it: "Phantom Power Supply for Guitars".
which showed how to supply DC power to an amplifier built in to a guitar via
the 2 audio wires.

I'm guessing it may not be too bad a starting point if you were sending DTMF
control tones as audio rather than digital, perhaps.
Then there's the cost of dtmf encoders & decoders...

And the bit of paper also has noted on it,
RS-232 Opto-isolator article December 1997 (Elektor Electronics),
PIP02 (David Tait) PIC Programmer, July\August 1998 (Elektor),
and part one of a series of PIC articles beginning July/Aug. 1994.

/David_v