There's no option to decline the call. However, if your phone is unlocked, you will be met with that green "Accept" button or that red "Decline" button when someone calls you. Though this ...

When your phone is locked, here's what an incoming call looks like: If your screen is unlocked when the call comes in, this is what you'll see: The elusive decline button really bums some people out.

Colin Gorenstein If your phone is unlocked, however, the screen that appears during an incoming call is different. You’ll see the two buttons, “accept” or “decline.” Colin Gorenstein ...