They are called gateways https://sr.ht/~nicoco/slidge/ https://biboumi.louiz.org/

You can do basically everything except multiuser encrypted calls (we use Mumble for this anyhow). But even then Jitsi (& proprietary Zoom & WhatsApp) are built atop XMPP for the backbone of their protocol using XMPP to negotiate connections before handing off for calls.