While Jitsi is open-source, most people use the platform they provide, meet.jit.si, for immediate conference calls. They have now introduced a "Know Your Customer" policy and require at least one of the attendees to log in with a Facebook, Github (Microsoft), or Google account.

One option to avoid this is to self-host, but then you'll be identifiable via your domain and have to maintain a server.

As a true alternative to Jitsi, there's jami.net. It is a decentralized conference app, free open-source, and account creation is optional. It's available for all major platforms (Mac, Windows, Linux, iOS, Android), including on F-Droid.