This app was removed from the App Store.
iOS iPhone Utilities
Got proximity sensor issues on your iPhone4? There's an app for that! Call Guard prevents accidental user input while on a phone call, putting an end to accidental hang-ups, mutes, speakerphones, etc. There is no other way to lock the screen while on a normal call, as the Lock button will End the call on OS 3.1 and later.
No more e-mails written by your cheek!
Once on a call, start Call Guard. The quickest way to do this after the first time is to double-press Home and fast-switch to it. Call Guard displays an attractive colorful screen with no interactive elements while on a call, thereby totally preventing accidentally pressing any numbers, hanging up a call, turning on speakerphone, muting the handset, or any other interaction that might be undesirable while on a call.
Call Guard does not do anything beyond provide a completely safe app to run to prevent accidental input. This is necessary because if it had features beyond that, they might accidentally be triggered while on a call (and thus not paying attention to the touches on the screen).
The only solution that really works!
Call Guard works better than launching other apps such as Calculator, because the "Return to Call" overlay that appears at the top of the screen during a call is also hidden by Call Guard. With other methods, it is still very possible to tap "return to call" and then you are back at square one.
When you want to interact with the touch screen again, simply quit Call Guard by pressing the Home button on your device.
Added a Setting (see Settings app on your device) to show a darker version of the backdrop while on a call. This reduces "face glow" when using the phone at night. The touches will still be easily visible.