Office Communicator Team blogged a really good post explaining how to schedule and join online meetings using Outlook Conferencing Add-in.
Scheduling and joining online meetings (Conference calls/Live Meetings) got a lot easier and powerful with the R2 release of Microsoft Outlook Conferencing Add-in. Microsoft Outlook Conferencing Add-in now provides dial-in numbers in the meeting invite to join the conference/live meeting by a phone.
Read more at source: http://communicatorteam.com/archive/2009/03/16/624.aspx
Update for Live Meeting Conferencing Add-in for Outlook: http://support.microsoft.com/kb/960164