cellular calls interception is something that has been available almost from the moment he invented the mobile phone. Although the GSM standard specifies that the transmissions must be encrypted, using devices called "Receivers IMSI (IMSI Catchers in English)" the authorities and intelligence agencies can intercept and decode calls while they are made (literally "air"), but these devices are quite expensive (we're talking about several hundreds of thousands, coming out to spend even a million dollars).
However, Chris Paget, a hacker (and researcher on security issues) developed a device (using items easily available and free software) that basically simulates a Radio Base Station (RBS - Radio Base Station in English), which is the antenna mounted on a tower that transmits the signal to cell phones, and with this RBS "false" that connect the phones because they can not distinguish a "real" orders phones to stop broadcasting code which can record the entire conversation. The cost of the device: U.S. $ 1,500 (which includes the purchase of a laptop, so the device itself is much cheaper than that.)
This method only works for outgoing calls, not incoming calls, which will be sent to voice mail (as if the phone was off) as long as the interception.
Although the device only works for calls in 2G (not 3G), you may send a signal interference on 3G so the phone will need to connect in 2G mode making it vulnerable.
Chris Paget gave a live demonstration at DefCon 2010 (held in Las Vegas in late July) and managed (with a mark of only 25 milliwatts of power is far less than that emitted by cell phones) to mobile of attendees at your presentation (indoors) be deceived. About 30 phones were connected to the RBS and these were false (to demonstrate that not only are security issues but also of privacy) a recorded message saying the call was being recorded (message that was sent by the same set of Chris) , all conversations were recorded to a USB memory which was destroyed after the show.
The most interesting thing is that the GSM specifications to determine if a phone is connected to a RBS that forces transmitted unencrypted conversations, a warning message should appear on the mobile screen informing the user of the situation. This warning is ignored by the phone and therefore does not inform users that they believe their conversations are secure. ------------
REFERENCES:
Wired: http://www.wired.com/threatlevel/2010/07/intercepting-cell-phone-calls/
