This is a very low cost and simple Arduino GSM and GPRS shield. We use the module SIMCom SIM900. It’s the cheaper module now avalaible in the market.
To connect this module to Arduino I make a PCB that include a LM317 some capacitor filter and no more. I performed a GSM library to controll easly the module. The GSM library is a modified version of the library of HWKitchen.
With our version we controll the module throw the pin 4 and 5 (so normal digital pin) and our GSM libray include also the NewSoftSerial, so you can easy control the module, send and read SMS, make call, control the GSM state ecc.
To download schematics, PCB and Library check the site.
Arduino GSM shield – [Link]