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?
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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."