How to Ignore a Call on the iPhone When it is Locked
When you get an unwanted call on your iPhone, you don't have to wait for the caller to give up or your voice mail to pick up to stop the ringing. When the iPhone's screen is unlocked, you can send the caller directly to voice mail by touching the "Decline" button. When the screen is locked, the "Decline" option doesn't appear, but it's still possible to ignore the call.
Press the "Sleep/Wake" button on the top right of the iPhone once. The incoming call will be silenced.
Press the "Sleep/Wake" button again. This will send the incoming call directly to voice mail. A missed call notification will appear on the lock screen.
Press the "Home" button and slide your finger across the phone to unlock it. This action will clear the missed call notification from your screen.