xmpp although some might think is dead, is doing pretty well. Recently conversations and siskin got native voip and on dino (linux) voip is on it's way. Gajim is about to release amazing version with new UI introducing workspaces and i think their voip works too.
All that on servers you can run on pretty much anything as it does not require almost no resources to operate. small light, privacy oritented, community drivem and battle tested solution.