From the people who brought you GV. This could be a future Google app in the making. At the moment it seems to be a start up backed by Google. The UberConference service blurs the lines between Google voice and Google Hangouts. Free and paid services available. $10/mo includes a DID number, Callback, and many other features.
https://uberconference.zendesk.com/entries/22579287-How-Do-I-Create-a-Conference-
Use Your Computer (Requires Google Chrome)
Select this to join the conference through a web browser, offering you HD sound quality. Encourage your participants to join through their computers for the best audio experience. They can still join using a standard phone, however, their audio will be of standard quality
Use Your Phone
Select this to join the conference using any phone and dial the conference line manually. If you're not by a computer, you can also just dial the conference line from any regular phone. If prompted, enter in the PIN as well. If you are using a Pro or Business account, you will have the additional option of having UberConference dial out to you.