I think you need the GSM / ETSI 07.07 and 07.05 specifications --- these describe the AT command set that many GSM devices implement. (Phones usually have some proprietary extensions as well, check for docs from Ericsson, I guess.) They're available publically on the net somewhere or other.
I don't think there's much of a standard for how you get a serial connection to the phone in the first place so that you can issue those commands, though. Newer ones might use a USB modem class or Bluetooth serial port profile, older ones probably have ttl-level async serial on some wacky connector, or an IrDA port.