I know its really stupid question but one of my friend told me that he can listen to the guy who calling on his mobile phone without his phone being the one the call is controlled by.
What ways could this possibly happen?
closed as off topic by Rory Alsop♦ Oct 1 '12 at 13:12
Questions on Information Security Stack Exchange are expected to relate to Information security within the scope defined by the community. Consider editing the question or leaving comments for improvement if you believe the question can be reworded to fit within the scope. Read more about reopening questions here. If this question can be reworded to fit the rules in the help center, please edit the question.
Google Voice allows Call Screening:
"Call screening lets you know who's calling your Google number before you accept the call. It also allows you options on how to handle your calls so that you can choose to accept it, send the caller to voicemail and *ListenIn*TM, or record the call."