In iOS 10 it seems it is no longer possible to hang up a call on your iPhone when it is locked. In past release, the controls to hang up a call would automatically appear when moving or waking up the phone, but now you simply see the number or contact you're calling and the call time. It seems the call control buttons have gone, and to end the call, you have to unlock the phone, go to the Phone app and end the call from there, which is a hassle.
When looking on the internet about this, a lot of people think this is a bug, but actually, it's not... Apple has changed the behaviour because of the new way to unlock your phone (pressing the Home button rather than swiping).
To end the call, tap the phone number or contact once and the familiar call controls will appear again, allowing you to end the call.