Without a server of some type available nothing can be entirely reliable on the severely broken internet we have to deal with today. Open source projects only provide the software. There has to be someone out there to provide money for the bandwidth/server.

The current SIP clients try pretty hard to do things right in spite of the brokenness. You could try:

* https://www.linphone.org/

Mumble works well with very cheap and available servers but audio only.

If you want something really minimal and capable:

* http://www.creytiv.com/baresip.html

https://riot.im has reached 1.0 and supports all the same features - except phones (Can be done with SIP integration).

Riot UI is (still) confusing for new users.

https://wire.com is also open source but much simpler.

There are also https://meet.jit.si and https://jami.net/.