There is also Ring (formerly SFLPhone), with mobile and desktop apps, however it's still in an early state. Ring supports SIP and its own protocol which is basically serverless SIP and nodes are looked up via DHT (as in torrents). Looks very interesting, is free software and uses Qt.
Also Ostel (https://guardianproject.info/apps/) is nice, multiplatfrom, free software alternative which uses SIP if I'm not mistaken.

Personally I cannot understand why new protocols are implemented when there are already SIP and XMPP, and what is lacking is good mobile apps. Current situation is horrible, with so many protocols (Skype, Whatsapp, Signal, SIP and XMPP) it's difficult to communicate. And yes, most of your friends will use Skype or WhatsApp (because it's from Facebook).