GSM Shield + FSK Modulation/Demodulation


I was thinking of transferring digital data using my GSM Shield (based on SIM900 Module). I know the GSM Module can do a data call (CSD call) but my network operator has blocked it (I guess so). Every time I try to dial a number for data call, it says "No Carrier" but for a voice call, it connects.

So I thought of transferring my data on the voice channel itself, using FSK Modulation to send 2 different frequencies for 0 & 1 (binary). I generated a waveform from my picoscope and gave it to the mic input of the shield and connected a probe to the speaker out of the shield just to check the feasibility of the setup.

I tried sending a square wave but received a sine wave, then sent a sine wave and received a sine wave. Although the frequency of the sent and received sine wave matches but there is significant noise and the frequency jumps in between to random values.

With the noise its not possible to demodulate the signal. I don't have much knowledge about tele-communications, so if anyone could figure out what is wrong it would be a great help.