If you need to call someone back quickly, you do not have to redial the number. In fact, don't bother digging through your phone app to tap the number in the call log. Instead, use this simple trick to quickly redial the number, whether you have an iPhone or Android.
Open your telephone app and then tap the call button at the bottom of the screen. As long as you have dialed the last number before you made the call, that number must be entered automatically in the field above. However, that is only half the battle. Make sure you tap the call button again to actually make a call.