Mumble. It's somehow funny when you get your software tips from a political party, but never mind. Mumble is something like Teamspeak (or also a distant relative of Skype group chats or Google+ Hangouts), only open source and for self-hosting. Clients are also available for OSX, Windows, Linux, and iOS. By the way, there are also efforts to integrate mumble as an alternative to Vivox in OpenSim as a voice system.