First of all I want to mention that Nokia C7 and Nokia E7 are very cool phones, I like using them and the new Symbian^3 operating system is fast.
Back to the issue: locking the screen during a phone call
Nokia C7, Nokia E7 and probability all Symbian^3 phones made by Nokia are designed to turn off the screen during a phone call (without usage of headsets). This is a required feature for touch screen phones that prevents the user to accidentally touch the screen during phone calls.
Phones have some kind of a proximity sensor near the internal speaker that turn off the screen when you hold the phone close to your ear. The problem with Nokia C7 and E7 is that if you are a normal person you will move/tilt the phone a little bit while you are talking.
The sensors are too sensitive or too few and even a small movement will light up the display and the touch screen will interact with your face. It happened to me lots of times and after putting people on hold/muting them and even terminating the conversation I found out a fix for this.
Maybe it's obvious to others but here's how to lock the screen of Nokia C7 and E7 during a phone call:
After dialing the number you need to slide the lock button (on the side of the phone). This will lock the display! You will not accidentally touch the screen with your face and you'll have pleasant conversations without interruption.