If you are using Microsoft Office Live Meeting on the Microsoft Office Communications Server 2007 R2 release, then you will find the Meet Now button as an Outlook Add-In that you can use from Outlook to create ad hoc meetings.
Also, in Office Communications Server 2007 R2 you can use the new Conferencing Auto Attendant feature to bridge together Computer Audio and dial in participants. Users who do not have access to a computer and want to join an Office Live Meeting audio conference can now call into the conference using a regular mobile, desk, or home phone. When scheduling an Office Live Meeting conference, the organizer can add a conference call number to the conference. Invitees can use any phone to dial into the conference and participate in the audio portion of the conference.