Great, now you just need somebody to rent a server for you.
That's where Discord won, along with being able to run in a browser for those who didn't want to fill their PC with crap comms software for one PUG run through Uldir.
Does it also do temporary passes so you don't have to give full access to people who only want to play alongside you once?
One issue I had with the Discord web client was the lack of push to talk. Anyone who raided with a Darth Vader will relate. I presume Mumble would be similar. You don't really want to give a browser full key logging access. Useful for listening in though.