Basically U can't use FSK coz,if u shift frequencies as we do in FSK for 1 & 0,u might interfere with other co-cell signals.
Originally in AMPS they used FM for voice,but nowdays in GSM when u have a BW of 200KHz per channel you don't want to use this shifting business at all.It is not efficient way to modulate the singal as far as your question is concerned.
Also I don't remember exactly but the BER is high as compared to other modulation schemes but not very sure of this.