Don't know about High Sierra issues but Linphone (http://www.linphone.org/) usually works very well. Jitsi Desktop (https://desktop.jitsi.org/) is a decent SIP softphone too but it's really ugly. Both are cross platform and open source, don't know about commercial software.

I wouldn't recommend native SIP softphones anymore, it's a pain to configure and run securely. The best cross platform media stack is the WebRTC functionality in modern browsers, and it usually just works perfectly with a VoIP provider or PBX that has a WebRTC frontend.

I've heard Bria 5 is pretty good.