Why does my Samsung Galaxy S3 make a weird beep beep sound when I'm on a phone call? It started yesterday when the call started.
Check the settings for your dialer app. Most dialers have some kind of "call in progress" beep that goes off every minute or so if the option is enabled.
This is normal when someone else is calling right at the moment you do this call.
I think it's called "call waiting", but the feature is usually non-free.
It's a feature called minute minder, and seems to be reset after an update. Go to Menu > Settings > Call > Call alerts > uncheck Minute Minder.