This is what I see happening on my One Plus 3T.
When I have my phone locked, and in my pocket, if I receive a call, the camera opens up which causes the lock screen to activate. When I get the phone out of my pocket, the phone either unlocks via face unlock, or I have to enter the pattern to unlock it, as the call fucntion is trapped behind the camera function.
As soon as I unlock the phone, the camera function stays open, and I have to close it, while doing so the call function disappers. (It never shows up as a running app). The call is still going on, my phone is still ringing and vibrating like crazy, and I have to drag down the notification dock, which does not come down until the 3rd try, and then click on the green phone sign for the call notification to answer the call.
All of this on a good day.
On a bad day, my phone gets locked (as in nothing gets displayed) as soon as I am getting the call, and I have to wait for the person to cut the call, before I can do anything with the phone.
If I have the phone unlocked, and I am doing something on the phone, I get the top notificaiton for the call, and I am able to answer the call without any issues.