Some users are reporting an issue with the proximity sensor on Apple’s iPhone 4. The issue seems to be that the handset won’t utilize the proximity sensor and the screen would turn on in the middle of the call, resulting in your cheek, when the phone is held against your face, pressing on the screen and causing accidental key presses since the screen isn’t off and de-activated. In a normal environment, the iPhone 4’s proximity sensor should be able to detect its proximity to your face and turn off the screen and turn on the screen once it moves distally away from you.
This issue has been affecting many users although some users have not been able to replicate the issue, suggesting that this isn’t a widespread defect like the antenna issue that has been plaguing the handset. However, some users have noted that they were able to rectify the situation with the proximity sensor by doing the following:
Many affected users report that either a Settings reset or a hard reboot fixes their problem. One Apple support thread recommends that users open the Settings app, then go to General -> Reset, and choose “All Settings” at the top (note: this will not erase your apps, media, or other data — that’s what the “Erase All Content and Settings” button just below this option is for. “Reset All Settings” just reverts any Settings preferences you’ve customized back to their factory defaults). One Twitter follower, Brian Partridge, reported that AppleCare told him to just use Reset -> Reset Network Settings, not all settings, and it worked.