Allegedly Freeswitch can be got to work, but it's complicated and even then, there is no guarantee that it will work consistently, and still involves running a server (unless of course the grid runs its own Freeswitch server, which would be feasible on some girds, but not on others e.g. OSGrid, as all but the Plazas are run on widely distributed servers around the world, and would thus need to run their own instances. Hardly anyone uses it, as usually people struggle with it and fail.
As Opensim has precisely one person working on the core code, it's highly unlikely that anyone is going to invest the time developing a simple plugin so that it works with Opensim, and in any event, Freeswitch, being Third Party Software means that anyone who wants to use it is pretty much on their own. From what I can see, it's about as streamlined to install as it's ever going to get, and whislt is doable, with patience, there is a lot of tweaking to do, all without guarantee of success.
From what you say, maybe Skype or Discord would be a better option for you as you aren't bothered about privacy, and aren't willing to fund an alternative. Discord certainly seems to be widely used by the gaming community.
It’s certainly possible that in the future we could have our own voice solution for Opensim, but it’s more likely to be Mumble based than it is Freeswitch simply because of the spatial element, which inworld is pretty much an expectation. It’s also more than likely that any such soution will have to be paid for, somehow.
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