IPhone won't let me deny calls?

When I get a call, it just says "slide to answer" without letting me deny the calls, so I have to answer then hang up.

On iOS 6, it show slide to answer and a little phone, that, when you slide it up, shows more options, such as accept / decline and more options.

On iOS 5, it only shows slide to answer. Tap the power button once to silence the call, twice to reject it.

or Just don't Slide it.