Specifically, the Desktop App needs to be treated separately from the Smartphone App.
Ideally, it would be best if software would know if you were active on the desktop app or not, and forward to your mobile app (assuming you were logged in and open to receive calls) if you were idle. Slack does this beautifully.
However, the easiest way to do this currently would be to allow users to set their desktop app to ring for a certain amount of time, then forward calls to their smartphone app.
As it stands right now, if I stay logged in to both the desktop app and the smartphone apps, I get calls in both places. This is quite annoying.