We initially tried TeamSpeak and Mumble, but found that they were too different from Discord. We then tried Stoat, but it was too unstable at the time, and sending and viewing messages was difficult, so we moved to Fluxer, where we have been for the last week. Both seem to be solid projects, but they are suffering from infrastructure issues due to the sudden high demand. Check them both out and see which one your group likes the best.