Well, we can use Skype (which has a number of restrictions), or we can use mumble becasue push to talk is a life-saver (I run a server). Cast a vote please!

I personally would prefer Mumble, as it allows for people to just idle around and hop in/out as they please without starting/joining a call. I'm also a huge proponent of push-to-talk, as I tend to be a heavy breather (no-one wants to hear that) along with slight rage issues. Just my 2 cents.