Avoid obtrusive incoming call window
From discussion at #209 (comment 146267)
Using some kind of popup for incoming calls (rather than the current fullscreen approach) can provide a nicer experience as it is unobtrusive and allows you to finish what you are doing before answering the phone.
If I'm in the middle of typing a message, for example, I can finish my sentence and send the message before answering the call. The current design requires you to immediately respond to the call mid-thought (which I think is against the original design goal of GNOME 3 to be distraction-free).
An example from the Ubuntu phone can be seen here (but, obviously the current app would be shown underneath, rather than a black screen): https://i.stack.imgur.com/mEGm3.png