There are several types of FSK (IIRC)- can you be somewhat more specific?
Also, try to insulate from external noises. I remember using phones fixed in rubber cups of a modem. If the fit was not tight, the connection went dead (disconnected).
My memory is rusty, but there are several dual tones that act as control signal for the modem (out of the 16 possible dual tones, only 12 are present on the keyboard).- perhaps google can help!!
What is the exact setup (some more details please!!)?