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