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Once upon a time
I got iphone 4s from 2012. It has been used for 3.5 year. In the beginning of 2015, the mute button started to behave abnormally. When switching to silent mode, you can find the screen showing ring mode automatically sometimes. A few second later, it turns into silent mode again. This is real annoying that the phone vibrates in your pocket but you find no income message, just a self vibrating iphone.
There are lots of cases that mute button malfunction among my friends. Almost every one of them using iphone4S has this problem, except for one who never uses the silent switch.