What opportunities exists to connect a GSM module to a Raspberry PI?
Which GSM modules are supported or can configured with little effort?
What software was used to dial and connect to a GSM Network?
You can buy a GPRS/GSM module for the Pi called the SIM900, from this website. It is also compatible with Arduino and Intel Galileo.
The same website also includes very detailed instructions to get the module working with the Pi, it also includes all of the code to get the module working. The software required to get the code working is called minicom. If your interested in the tutorial look here.
You can always buy Raspberry Pi GSM/GPRS Shield. Try not to buy GSM module with only RS232 port as I found it lot difficult to communicate. There are many modules which has USB-UART port for power input and same for communication with PC or Raspberry Pi. You can buy such from this GSM/GPRS Shield - SIM800 .