Can't tell about WebRTC or browser-based. When it comes to software, I've tried everything from the Wikipedia list and some more.

Most are either too buggy (Linphone) or closed source (many) or not multiplatform (so you can't talk to WindowsXP-user grandfather or vice-versa) or development stopped (Twinkle) or has no encryption (Ekiga).

The only one who passed was Jitsi.

It has more than 10 years of development, it's free / open source, very usable, multiplatform. It also accepts encrypted voice over XMPP, not only over SIP.

Look here - https://jitsi.org/Main/Features