Comment 4 for bug 1419645

The incoming call screen is currently a screen in the Phone app. Accepting the call switches to the app. How, then, could the desired behavior be implemented? Unity would have to remember where in the stack the Phone app was previously (if it was even running at all). The Phone app would have to tell Unity that the call was over. Then Unity would have to move it back to its previous position in the app stack. That's a lot of special-casing, and it's hard to see how any other VoIP app could do the same. (And on a PC where the app wasn't full-screen, that shuffling would look weird.)

If the call window was a parentless dialog instead, whether full-screen or not, it could float on top of whatever app you happened to be using at the time (even if that happened to be the Phone app). Once the call finished, the dialog would go away, and you'd be back where you were without any app shuffling.