Zoom
There are many other open source solutions that might be better, but you should try out element-call (github).
Its the system used for calls by matrix/element, but it can also be hosted standalone as just a video conferencing tool.
You can test it out without an account here. https://call.element.io/
This site is sort of a demo i think, but i havent run into any limitations that would prevent you from using it for actual conferences.
I find it quite usable and the software can scale to very large calls if needed.
Im biased but i would of course also just recommend matrix in general to replace teams too. Ive been using it for years and its improved a lot. Its definitely still not entirely feature complete though, i have to admit.