People have had success with Mumble servers. The main issue seems to be the lag between each person speaking, so they recommend using control words ("Roger, over"). Take a look at https://guardianproject.info/2013/01/31/anonymous-cb-radio-with-mumble-and-tor/ I've also heard some chatter about running Jitsi over Tor. It shouldn't be much worse than the few seconds delays you see when someone is being interviewed by satellite from a remote location. I think it's an acceptable trade off for the privacy that Tor provides, compared to routing your communications through open clearnet infrastructure. As opposed to the LE backdoors that they will download to your unrooted phone?