Why does my iPhone screen go blank on a call?

The iPhone will only go blank if you deliberately press the sleep button to shut off the screen during your call, or if you trigger the proximity sensor. The proximity sensor is designed to turn off the screen during a call if the iPhone is close to your ear. This prevents accidental pressing of buttons on the screen and saves the battery, as well as keeping your ear cool.

When you're connected to a call but the iPhone is not next to your ear the proximity sensor still works. The touch screen will go blank briefly if the proximity sensor detects something such as a hand or finger. The sensor is located at the top of the iPhone next to the ear speaker.

Normally you can't see this happen as the iPhone is next to your head on most calls. When using headphones or making a call with the speaker option the proximity sensor is not active, so the screen will always remain lit unless you press the sleep button.