Hi
Well, I can't help you totally but I think I can give you some help, because time ago I made a similar system using a voice rf link on radiocomunication equipment.
My idea was similar. I will send over a voice channel dtmf tones to represent digital data.
On the reception side, I took the signal from voice radio channel with a mic and then sent it into a standar dtmf decoder. Seems easy, but not.
The problems:
first, dtmf standar chip decoder are design to fit with the ITU-T recomendations. So they don't accept more than 2.5% signal error( check this on the datasheet). On a rf channel, signal suffers transformations and then, when we use a mic, depending its quality, the audio signal is affected again and then, on the amp stage, etc, etc. So when finally come to DTMF chip, the tone received could not fit with the original one. Consecuently, dtmf chip don't accept it.
My solution was use a pc soundcard to record signal and then using MATLAB analize it on the frecuency plane. Some DTMF tones arrives with great error, so I made a Matlab's program fast fourier transform based and used it to decode the signal. it Worked fine on PC. After that, I put this program into a dsp and run the system again. Everything works now and I can send, at low baudrate, short digital information over a voice channel on HF radioequipment.
I think you are having similar problems, try to check.
check too if when you press a key, it reproduces exactly a dtmf tone.
Hope it can help you a little.
good luck