The GSM SIM800 connects your Arduino to the mobile network using the GSM/GPRS wireless network. Just connect this module to your Arduino board as shown in figure, plug in a SIM card from an operator offering GSM/GPRS coverage and start controlling your world. You can use it to make/receive voice calls (with an external speaker and microphone circuit), send/receive SMS messages, access the internet,etc.
For reading more about GSM AT Commands please visit:-
// put your setup code here, to run once:
const char ctrl_z=26;
// put your main code here, to run repeatedly: