Surely this only happens because someone wasn't looking at the screen while they were making the input. They could just as easily miss whatever animation is attached to the number changing. The real solution here is to provide some kind of haptic feedback (phone vibration) or to play a sound.
Can you back up this opinion with any kind of coherent argument? Because the problem is "people can't see the screen" so I feel like displaying something on the screen might not solve it. By contrast, you would definitely know if the phone buzzed when you clicked a button. Anyone who's ever used one of the iPhones with fake home buttons on them can attest to this fact.
