My biggest issue with calls on Signal is that if the app crashes, the call disconnects. This has happened to my on my up-to-date iPhone (10S) if I'm on a long call and get another Signal message, which means I need to unlock the app again.

On the desktop, I really wish that you could resize the contacts sidebar to just show the sender's icon, as you can in iMessage, but the maintainers don't seem to be particularly interested: https://github.com/signalapp/Signal-Desktop/issues/2510