Ring.cx run server-less. It use the same technologies as Bittorrent (DHT tables) to find and connect users. You can also self host a server such as Asterisk or

Ring.cx run server-less. It use the same technologies as Bittorrent (DHT tables) to find and connect users. You can also self host a server such as Asterisk or Freeswitch in a VM if you like, this is supported too, so is inter-operability with commercial VoIP providers.