Weird problem: Phone rings and/or vibrates but the screen doesn't change. All you can see is the home screen. How can you answer the call? You can't.
Well you can but you need to know what you did!
In my case, I am on the Three mobile network, which is, basically, terrible around where I live and work. They have an app that uses the Internet for calls and texts if you are not in mobile range. If you are, all kinds of pain occurs because texts sometimes go to the app and not the phone etc.
Anyway, like many annoying apps, decide that they can fill up your notification area with a permanent message saying that Wi-Fi calling is ready. Amazing!
Anyway, I had disabled notifications for the Three app to get rid of that annoying message but the thing I hadn't realised is that it will also disable the incoming call notification. For some reason, the phone still rings and vibrates but you can't do anything.
I had to re-enable notifications and then, of course, I can answer the phone! I also have to live with the annoying permanent notification.
The same has also occurred with Google Dialer according to forums but I don't have myself.