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User records a voice or whistle and app responds with a recorded message or whistle or any sound the user wants!
If you have lost your phone and it might be down the sofa! then give it a shoutback.
example: user : "where are you phone"
phone answers:" I'm over here" !
I understand there will be a bar on the iphone screen. The app must have an option to turn off though or users will get annoyed.