There is no such thing as a private conversation on a mobile phone, not even one sided privacy, unless there's no-one within 5-6 meters indoors or 2 meters outdoors (depending on ambient noise levels).
In particular, I can easily hear the far side of the conversation when someone on an iPhone is on a call in the same room as me.
Don't even get me started on people that use speakerphone in public. (photo by me, in a typical mall scenario)
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