Mumble is a voice chat software. Primarily intended for use while gaming, it is suitable for simple talking with high audio quality, noise suppression, encrypted communication, public/private-key authentication by default, and "wizards" to configure your microphone for instance. A user can be marked as a "priority speaker" within a channel.
FreedomBox includes the Mumble server. Clients are available for desktop and mobile platforms. Users can download one of these clients and connect to the server.
If your FreedomBox is behind a router, you will need to set up port forwarding on your router. You should forward the following ports for Mumble:
TCP 64738
UDP 64738