Getting the contacts/ storing contacts to SIM card I have the GSM shield and is there any ways to get the stored contacts on the SIM card? It seems that the GSM library does not support writing data onto/ reading data from the SIM card?

My aim is to store the SIM card's phone number and some variables onto the SIM card itself, such, I would not need to change the program from time to time.

All I need to do is enter all the variables in the SIM card via any mobile phone (perhaps by storing the variables as contacts, as phone book function should be very common on phones).
What have you researched so far? Following the link you have given for the shield, about the third line of the description is a reference to the AT Command Set used by the chip.
I just did a couple of quick searches through it and found:

2.2.4  ATD&gt;&lt;n&gt; Originate phone call to number in memory
3.2.26 AT+CPBF Find phone book entries
3.2.27 AT+CPBR Read current phone book entries
3.2.28 AT+CPBS Select phone book memory storage

probably a good place to start.