I can comment barely on each. I'm currrently using Ring.
* CSipSimple, Jitsi, and Linphone: They provide SIP support. SIP is somewhat old but is similar to telephone communications, although it's very heavy on the Internet usage. SIP *does-not* connect to telephone network, but there are services (and software) that allow you to do so (I can't elaborate more on this since I don't use SIP anymore). * Jitsi and Pidgin: Provide XMPP support. XMPP is by default text-only, but overtime, there are extensions that where published (including some that add video and audio support). Even though these extensions are also standardized like the main XMPP standard, they're optional (the service provider can choose what to support), and there's generally no standardized fall-back (e.g.: If the receiver doesn't support audio, make the sender send a temporary link that has the audio recorded, although this would only work with a push-to-talk solution, but XMPP's audio extension doesn't seem to be push-to-talk). * Tox and it's clients: I can't talk about the functionalities, but there are some past happenings that scared away the whole Tox community, and because of this, I can't really tell how well it is, and how much it'll endure. * Ring: Seems to function like Tox but it started to publicized very different than Tox project. While Tox choose not to appear in public conferences exclusively related to free/libre software movement, Ring has taken the first step during LibrePlanet 2016 (which is one of the few conferences that are indeed aligned to the free/libre software movement). * There are other software and projects that you can find to be related to communications, but *some* of them are browser-based. I have been in contact with few of these, but my experience shows me that, while their software is free/libre in the level of code, licenses, dependencies and such like (and so deserve to be listed in the Free Software Directory), their mentality/ideology is too much "open source"-only.
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