I have played with a few programs and think that it will work, providing one is prepared to pay.
At this stage I think I will use Voxwire http://www.voxwire.com/
It costs US$30 a month for 10 seats which is affordable.
A relatively new kid on the block is Wolf Communications it's still a beta version but it's free for a 3 people room. Not that this is of much help for teaching, but it's not bad to have a bit of a play and get the feel for it. http://chat.wolf-voip.com/my/wolf/#
The issue for me is now, how I can incorporate this technology into Moodle?