We use Mumble exclusively.

The server is at hotmess.mumble.cc and the port number is 2463.

Anyone online can tell you the password. Download mumble and figure it out before the raid.

The password occasionally changes on its own, and guild members are registered to the server (giving them access without the password–Mumble will remember who you are and allow you to enter even when the password changes).

This means you can freely invite pugs to use the Mumble server and freely give out the password. In case there’s a troll, there are channels which only registered users have access to.