Ola! I'm new into the arduino's world. I wanna build a remote control for turn on/off my radiators! I would use a old phone or similar... However cheap... Do you have some ideas? 
I'm quite good at electronics but gsm system is totally new for me...
Please let me realize my projects! Thanks guys!
Welcome to arduino world. Look into my new site power7.net . I had make a project for sms remote control with an old mobile phone as GSM modem.
I see, but there's only the sketch and nothing more... I wanna understand how the phone can communicate with arduino and what is the necessary hardware. Can you help me? Where did you learn AT commands? What are these exatly? 
Thank you for your time and patience, as you can see I'm a newbie in this field... But I wanna understand 
Ad maiora semper
For hardware you need an old sony ericsson or other type old cellphone ( i use Sony Ericsson K700i) and a cheap cable from ebay.I cut it and connected to arduino serial.Look into photo page inside my site.One image,one thousand words...,AT commands from google and manual for your phone.I update my site with more infos about sms remote control.I think is a cheap and easy way for remote control from anywhere in the world!!!