You might be surprized at how many people listen to cellular phone conversations. All you need is a scanner and obtain the frequency that the cell phone operates on. Which most cell phones operate in the 900mhz frequency range. Most likely that is how he is doing it.
Used to be able to do the same thing with cordless phones at one point and time.
Now wether or not it is legal to do is another thing. Which I can not answer that question.

