According to Engadget and Wired, hacker Chris Paget fooled 17 GSM cell phones into believing that is $1500 kit was a GSM tower. He was able to record calls with his kit, which included a laptop and two RF antennas.

He says that the purpose of this was to reveal a flaw in the 2G GSM network, which will redirect your cell phone to the nearest tower that is pushing out the greatest signal. His laptop and the antenna were being used as a fake "cell tower". “As far as your cell phones are concerned, I am now indistinguishable from AT&T,” he said. “Every AT&T cell phone in the room will gradually start handing over to my network.”

Paget says that GSM is broken beyond repair, and the best thing to do is just turn it off. I don't think that is an option for AT&T and T-Mobile. Customers can stop hackers from doing this by just switching on their 3G, if that is an option in your area, but he said he could block 3G, causing your phone to turn to the nearest 2G signal, which would be his fake tower.