Compared to other modules that we have used so far, the GSM module is much larger in size. There are many pins that can be utilized for various purposes. However, for basic telephony, very few pins need to be connected with the Arduino:
Remember that you will find multiple GND pins on the GSM module board. If the incorrect GND pin is used, then your module will start getting overheated and will eventually burn out. For basic telephone use, the GND pin that is located beside the TXD and RXD pins.
If unsure of the correct GND pin, then there is an easy trial and error method to find out the correct GND pin. After powering up and connecting the module to the Arduino, if the SIM chip starts getting hot then you are most probably using the incorrect GND pin. The chip should not become hot if the correct GND pin...