iPhone - How to decline a call when locked

I recently changed my Android phone and decided to move to Apple, there are multiple reasons for doing this, which I might put in another post. One thing that took me sometime to find out was how to decline an incoming call when the phone is locked. Basically it is very simple, just double press the lock button, that is the one on the top of the phone. This is not as intuitive as other functions and I believe Apple could improve on this and make the option available on-screen, but for now this is the only way I know of.