Dial-in Conferencing is a new audio option for audio conferences that are hosted on Office Communications Server 2007 R2. The dial-in option provides a cost-efficient replacement for your company’s conferencing provider service. It also allows conference call attendees to join a Communicator conference call or a Live Meeting that uses Dial-in Conferencing for the audio by calling in from a phone or mobile device. The dial-in option can be used for scheduled Communicator conference calls or Live Meeting. With Dial-in Conferencing, members and non-members of your corporate network can join a conference call without the need for a conferencing provider.
– definition from Microsoft