Modulation and allocation in GSM900
GSM since it's first day has been digital (Fat32 is incorrect). That means you can't send analog audio. Luckily, analog audio had already been phased out in the 80s, in telecom.

In digital telephony, audio was digitally sampled at 8 kHz, 8 bits per sample, for a 64 kbit/s signal. If you has an ISDN phone, you got this quality at home, but it certainly was used inside and between the central phone switches.

GSM built on that 64 kbit/s idea, but used audio compression. The first codec used was GSM Full Rate, which achieved 5x compression for a 13 kbit/s net data stream.

GSM furthermore divides every frequency band into 8 slots, which means that phones can only transmit short 5 ms bursts and then have to wait for 7 other bursts from other phone users. Band spacing is a mere 200 kHz; a typical carrier gets 5Mhz blocks of spectrum which means 25 bands and 200 slots total.