backlight dimming too fast when on a phone call

Can someone tell me where this setting is at? If I'm on a call and the top-level, visible screen is the keypad (and I have the speaker turned on), it seems that on a random basis the screen light (backlight?) will turn off in less than 5 seconds. This is a little bit ridiculous, because it requires me to touch the screen every 3 seconds, when the screen fades to 10% brightness before it goes completely dark, to keep the light on when I'm pushing keys to get through AI systems in the corporate world. When the screen is black, obviously the keys are not visible and pushing the screen does not transact anything. Help anyone? thanks.

Adam