UPDATE: Check out the new iPhone 4.1 Software update which solves this for good!
Been having a problem with your shiny new iPhone? Yeah, I did too. My face kept triggering the speaker button when I was in the middle of a call, almost every time! Now because I’m left handed, the speaker was my problem – I’ve seen elsewhere that people’s cheeks keep pressing the mute button.. I’m assuming those people are righties!
Why is it doing this?
Well it seems down to the proximity sensors at the top of the device not detecting your face properly. It still works, as you can see if you ring voicemail and cover the top with your hand, but it’s not working quite right.
I’m 99% sure that this happens because you used a backup of your old phone, whether a 3GS, 3G or original iPhone, when loading up your iPhone 4 on iTunes. There are some kind of settings for the proximity sensor stored, and they don’t match up right – maybe because the sensors have been relocated in the latest iteration of Apple’s phone.
How do you fix this?
It’s pretty simple – there are two ways.
1. Wipe your phone completely, Settings->General->Reset->Erase all content and settings, nwo plug your iPhone into iTunes and set it up as a brand new phone. Yes you will have to sort everything out again. 
OR
2. Just erase the settings. Settings->General->Reset->Reset All Settings. This won’t lose any of your content, or your app positions/folders. It won’t delete your email. So far, I’ve noticed all it’s done is remove my saved wireless network settings, cleared my ringtones, and wiped my wallpaper. Not so bad if you ask me! Since I’ve done this, I’ve not had the problem!
Ofcourse, YMMV, but I hope this helps someone!
