It’s not overly obvious how you can reject or at least silence the ringtone on an incoming iPhone call when the iPhone is locked.
This may be a basic usage tip an often ‘googled’ one nonetheless.
When an unlocked iPhone rings you’ll see two options, Answer or Reject which is fine but on a locked iPhone a Slide to Answer option at the bottom of the lock screen is all that’s visible.
To silence the ringing locked iPhone press the sleep/standby button at the top of the iPhone or to reject the call altogether press it twice.
This works all the way up to iOS 4.3.3 and probably after that too.