I used iptel.org at first, then I adden account fromm ippi.fr because they have a bridge to the telephone network you can buy minutes on. I never ended up getting any minutes though so I can’t say anything about how well that works. Recently I got an internet connection with an sip phone line built in so I mostly use that now.
Voice works but I don’t know anyone that uses sip to communicate so I have 0 experience with video and desktop sharing. I never got those working on xmpp either but I might just have them set up wrong and I don’t know many people anyone who use xmpp either. It’s sad really, it seems like we are in a period where internet communication technology is going backwards, at least outside of the walled gardens.