There are some good options posted above. I have also tried Jitsi a while
back, with account at Ostel, and it worked well, but the setup was a bit
complex. Ostel website for a free SIP account: https://ostel.co/
I second Jitsi Meet for a simple solution. It runs in a browser, works well
and is easy to use. I found it to work better than Firefox Hello, too. I only
tried the live web instance, but I believe you can use it on your own server,
too. You need to make a link and share it to start a conversation, which may
or may not be feasible for you. I think it may be the best solution for now,
though. Website: https://meet.jit.si/
I think the true replacement for Skype and similar programs will come from
the secure and decentralized alternatives, prime examples of which are Tox
and Ring. They are both similar, but neither is complete and ready for mass
adoption. There are people who use them without many problems, though.
Tox is probably more advanced feature-wise. When I used it, it usually worked
well, and I liked the UI. What I had my doubts about was the approach of the
developers/maintainers towards the project, which seemed quite chaotic. The
organization also went through some internal problems, but hopefully those
are resolved by now. Tox consists of a core that many clients have been built
around by many people. Most of them seem dead now, but the two that kept
around are uTox and qTox. Website: https://tox.chat/
Ring is still lacking some features compared to Tox, but the development is
led by a company who already has experience with VoIP software development.
They seem to take a more serious approach to the development, and have more
concrete plans for some of their work. There is an offical client for GNOME
and unofficial for KDE, but of course they may be used in any environment.
Website: https://ring.cx/
Both Tox and Ring use a long string of characters you need to give the other
person to add them/call them. You only need to do that once, but there are
efforts to simplify it. Someone people made websites where one may register
to assign a name to their Tox addresses. There's been talk about integrating
some of those websites into a client, but I don't know how that ended. Ring
is now working on implementing searchable profiles directly on their
decentralized network, so the feature is expected to come in close future.
