I remember a few months ago I was doing some personal research about cell phone security and what I can remeber is the different cell phone technologies, which make it harder or easier to listen in on convos.

The Technologies are:

FDMA
TDMA
CDMA

According to the How Stuff Works they work as follows:
FDMA puts each call on a separate frequency.
TDMA assigns each call a certain portion of time on a designated frequency.
CDMA gives a unique code to each call and spreads it over the available frequencies.

This website http://www.owlnet.rice.edu/~elec301/...a/compare.html has some charts that show how the technologies work.

As far as I know most phones today use GSM (Which uses FDMA and TDMA) or CDMA.

And with that said here is the difficulty of someone being able to listen in on your conversation, depending on what technology your phone is using, according to http://www.moonraker.com.au/techni/cdma.htm

With FDMA a signal can be detected simply by using an off-the-shelf scanner. TDMA detection is more difficult but not impossible for the expert. Obviously CDMA privacy is much better. It is very difficult to locate calls in the noise stream let alone crack the code to gain illegal access to the data you want.

CDMA tapping is possible, although it has it's limitations read on: http://www.antionline.com/showthread...hreadid=249360