Here is the Seeedstudio GPRS shield:
The wiki page says “Make sure your SIM card is unlocked”.
What does unlocked SIM card mean? Does one have to get some specially made/hacked SIM card to make it work? I was trying to get a prepaid T-mobile (USA) phone with SIM card to send SMS with the shield. Can someone with experience using this shield give me some answers? Thank you!