Thank you for using Apple Support Communities.
I see you are noticing that when pressing the lock button, the call is reset or rejected. This is the current behavior of this button.
From the iPhone User Guide, https://help.apple.com/iphone
Decline a call and send it directly to voicemail. You can:
Press the side button or Sleep/Wake button (depending on your model) twice quickly.
Press and hold the center button on your headset until two low beeps confirm that the call was declined.
Tap Decline (if iPhone is awake when the call comes in).
Note: In some areas, declined calls are disconnected without being sent to voicemail.