I think [@Mer__edith](https://mastodon.world/@Mer__edith ) might know (or the people behind Signal (npub1rkw…5q62)).
One possible alternate videoconferencing platform (I realise is not Signal) is Jitsi. Haven't tried it with a largish group, but I did try it 1-1 and no sign-up was required.
You set up a meeting, then got given a link you could pass to the other party.
https://meet.jit.si/
That said, I'm interested in whatever others suggest. Video conferencing is hard to self-host because the bandwidth and CPU requirements means VPS hosting gets expensive and home Internet connections are rarely fast enough.