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.
Is a mobile phone number not required for a Google account? In many countries, including all EU ones, you need to authenticate yourself when ordering a SIM card. This makes your phone number your personal ID. Your Google account is connected to your person, and what you do on Jitsi after logging in with your Google account as well. It’s easier to track back to you that way.