Currently Dimdim offers audio integrated into the meeting using VOIP (Voice over Internet Protocol). The presenter and attendees require microphones & headphones to talk.
We have partners who are audio conferencing companies and we can put together a package which includes telephone calling too. If that interests you please email sales@dimdim.com We also plan to offer it as an integrated feature soon.
To more information about Telephone Confrencing please go through this following Knowledge Base Article
http://help.dimdim.com/activekb/questions/142/how+to+start+conference+call