I have a suggestion for when making calls. It may seem similar to other suggestions but is different. So my suggestion is to add an option where instead of the whole screen displaying who your ringing rather do a similar job to receiving calls. When receiving calls the whole screen doesnt display when you are using the phone.
I am making this suggestion because when I was driving and made a call via bluetooth from my car, the maps disappeared under the calling screen and whilst driving you shouldn't be using your phone as it very dangerous. Hence the reason for this minor but effective change.