Weird title I know. But nearly every client I tried sucked in its own unique way. After a lot of trying around I found that gnome-calls with gnome-contacts works the best. But I use KDE, so getting it to work was a little annoying but doable. And no, there is no KDE alternative that is not unmaintained or no proprietary.
But why gnome-calls? Every other program is either proprietary because this type of PC calling is mostly used in businesses and what do company's love more then bloat, proprietary mandatory and bad software, and most people don't use phones anymore so most programs are either over-engineered and way to complicated or outdated beyond usable. Thanks to Gnome Mobile and Linux on Phones (And to some extend PostmarkedOS and UbuntuTouch) gnome-calls and gnome-contacts are maintained very well. plasma-dialer should also work but most distributions don't have it because it is only meant for Plasma Mobile.