From what I've read sounds as though the microphone module on the dock connector and/or proximity connector needs to be replaced.
I've worked on several where I thought the proximity sensor was out, but, ended up being the mic module having trouble switching (and it is a cheaper and faster fix).
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I've also came across a couple of jailbroken iphone 4's where the jailbreak caused the same issues.
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I've also came across a couple of jailbroken iphone 4's where the jailbreak caused the same issues (cydia).
For reference, here is a simple of mic locaitons, etc on the iphone 4:
(note #2 and #10, both are microphones, but have different functions)
From what I've read sounds as though the microphone module on the dock connector and/or proximity connector needs to be replaced.
I've worked on several where I thought the proximity sensor was out, but, ended up being the mic module having trouble switching (and it is a cheaper and faster fix).
I've also came across a couple of jailbroken iphone 4's where the jailbreak caused the same issues.
For reference, here is a simple of mic locaitons, etc on the iphone 4:
(note #2 and #10, both are microphones, but have different functions)
http://ipod.about.com/od/introductiontotheiphone/ss/Anatomy-Of-Iphone-4.htm