If you need to quickly type a single number on your iPhone, then tapping the number key (to switch from the alphabetic keyboard to the numeric keyboard), followed by tapping the number, then tapping the alphabetic keyboard key (to return to the alpha keyboard) is a long-winded process involving three taps. However, this can be shorted to one tap and drag. Tap and hold the number key (you will notice the numeric keyboard appears without needing to lift your finger from the screen) and then without lifting your finger drag across to the number you wish to insert and then release your finger. That number will be typed/inserted and the keyboard will then flick back to the alphabetic keyboard without any further taps.