Sagem Hilo GPRS Shield - SIM problem

Hi, I'am trying to work with the following shield: http://www.libelium.com/squidbee/index.php?title=New_GPRS_module_for_Arduino_%28Hilo_-_Sagem%29

It responds, but keeps telling me that no SIM card is inserted (+CME ERROR: 10) I also tried 2 more cards, but without success. Two days ago I saw the shield working at a friend of mine. Do you know about this error? Thanks for your help!

The third card actually worked and is from the same provider as the other card it worked with. Is there some kind of netlock for this device?