Ask HN: What VoIP solution do you use for remote pair-programming?

Next my team is splitting between continent. To prepare for this move, we started using tmux (I highly recommend Brian P. Hogan tmux book (http://pragprog.com/book/bhtmux/tmux). While tmux is awesome and it's a shame we haven't used until now, Skype and Google Talk's connection isn't stable enough for an 8 hour pairing session, we need a more stable solution.

I've heard about Mumble (https://github.com/mumble-voip/mumble) but haven't yet tried it.

What VoIP solutions do you use for pairing?