This is the telecon infrastructure used by the German Astrophysical Virtual Observatory. Others are welcome to use it within reason. If you find yourself heavily using it, consider self-hosting (it's not hard).
There are three components:
Note that depending on your mumble client, the chat may not be obvious at first. To send a first message, right click on your channel name and select “send message” (or equivalent).
Recommended mumble configuration includes: check off Configure/Text to Speech; in Configure/Settings' Audio Input tab, at the Transmit field, select “Push to talk”; again in Configure/Settings, go to the Shortcuts tab, click on the existing shortcut and hit the right Control key (that's a key that doesn't usually get in the way of whatever else you want to do with your computer).
A bit more on the thoughts behind all this is on a post on GAVO's blog.