How to disconnect a phone call after pressing return button

I have an iPhone SE and have an irritating problem. Sometimes, after I finish a call, instead of hitting the red handset I mistakenly press the round button below (return button?). The screen goes blank and I cannot return to the dial screen to press the red button to end the call. This is particularly vexing if I have just left a phone message. I am an old guy and keep making this mistake, how do I return to the red phone symbol to end the call?

Thanks,

Gary