How to reject a call on the iPhone when it's locked

This had me beat for a while.  Such a trivial function and yet something I'd not yet discovered. 

When you receive an incoming call on a locked iPhone, you have the option to 'swipe to answer' and that (seemingly) is about it.  To date, what I've been doing (foolishly) is to just switch off the ringer switch, which causes the handset to continue ringing (but muted and still vibrating). Turns out there (obviously) is a way of rejecting a call and bouncing it on straight to voicemail.

So...how do you do it.  With the phone locked, you only have access to three actions that you can execute:

1. Press the Home button
2. Swipe to answer
3. Press the top/power button

The secret is in the last option.  It turns out that:

Pressing the button once will mute the ringing - for THAT call only.  This is handy since I can't remember how many times I've switched the mechanical ringer to the mute position and then forgotten to switch it back on causing me to miss calls. Not very convenient.

Pressing the button twice in succession will do exactly what we want. That's it.  Simply pressing the top power button twice will reject a call on the iphone while its locked. Have fun using this simple function which is so simply on all other handsets I've used in the past.  I have to say, given Apple's usual forethought and attention to detail, I'm disappointed this isn't more apparent. But, now we know...