So what's a good open source replacement? Jitsi, Jitsi Meet, Linphone, Ekiga, Jami and a bunch of others look okay [1], but it's hard to say how easy they are to use.
I've used Jitsi Meet with ordinary (that is, non-IT) people without trouble.I've also recommended it to elementary school teachers, who have been happy with it. It works with desktop browser, on mobile it offers a minimal app installation. Self-hosting is quite easy with their apt repo.
[1] https://en.wikipedia.org/wiki/Comparison_of_VoIP_software