When you type the letter or number are you holding down to long? If the phone is lagging and maybe you think its unresponsive you might be applying to much pressure. Otherwise can you type normally from any other screen, i.e. text message?
I have tried to push it lightly but when you push a number you can hear three tones when you push the key. The keys work fine while typing in a text message. It is only a problem while trying to dial from home screen.