Phone keypad turns white and key tones don't work?!?

I've noticed a really weird thing ever since upgrading to the new iOS. Sometimes, the keypad of my phone app turns white, and then if I press the numbers, I hear their tones but the phone system on the other end does not! I was in one of those automated help line things, and they said "press 3 to be connected to ..." and I pressed a bunch of numbers over several minutes until I realized that only I was hearing the tones, they weren't doing anything on the other end. After quitting and restarting, I had my normal black background keypad and everything worked fine - the buttons operated the options on the system on the other end like normal. Since then I've noticed this white keypad comes and goes and the phone acts differently when it's up. What am I missing - what is it doing when the keypad of the phone app has the white background?