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I can't out what makes it go away. Very annoying.
I would suspect a damaged mute switch or volume controls.
Doing basic troubleshooting of reset, restart, restore would be a good place to start. If none of that resolves, take it to Apple for evaluation.
I took my phone to the apple store and they said to restore the phone as it is a software issue. The reset and restart and resync did not work for me. Restore is next. I hope this works. The Apple Store ran tests on my phone and they did not find anything. One salesman said that my phone case may be making my phone too hot.

Reply something about ringer symbol stays on my Iphone 4s screen. It pops up randomly and won't go away. else having this happen or Post a New Thread