How do I simply dismiss an incoming call?

(I am a first time iPhone user so please pardon me if I am asking something stupid)

I wasn't able to dismiss a call by double pressing the power button. So I found out that it is because I have "Prevent lock to end call" turned on.

Now, I DO want this setting on. I mostly take calls using headphones and sometimes these calls are long. So it makes sense that I don’t want my screen to be on a 2 hour long call, right? However, for some arbitrary reason, turning this setting on doesn't let you dismiss the call by double pressing the power button and there is NO WAY to dismiss a call without this.

So I either sacrifice

1. Not being able to dismiss a call and let it ring till it gets dismissed automatically (like the first ever telephone)
2. Keep the screen on for the whole duration of call

Is it a known thing? Or is my iPhone (or me) tripping?